docs: add collaboration to FEATURES, ARCHITECTURE, and OpenAPI spec

- Add note sharing section to FEATURES.md
- Document collaboration access control model in ARCHITECTURE.md
- Add collaborators and user search endpoints to openapi.yaml
- Regenerate API.md

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/API.md

@@ -321,6 +321,123 @@ Permanently delete a note
 
 ---
 
+## Collaboration
+
+### `GET /api/notes/{id}/collaborators`
+
+List collaborators
+
+Returns all collaborators for a note. Requires access (owner or collaborator).
+
+**Parameters:**
+
+| Name | In | Type | Required | Description |
+|------|----|------|----------|-------------|
+| `id` | path | string | yes |  |
+
+**Response:** `200`
+
+```json
+[
+  {
+    "userId": 1,
+    "displayName": "displayName",
+    "email": "email",
+    "addedAt": "2025-01-01T00:00:00.000Z"
+  }
+]
+```
+
+---
+
+### `POST /api/notes/{id}/collaborators`
+
+Add a collaborator
+
+Owner-only. Adds a user as collaborator on the note.
+
+**Parameters:**
+
+| Name | In | Type | Required | Description |
+|------|----|------|----------|-------------|
+| `id` | path | string | yes |  |
+
+**Request:**
+
+```json
+{
+  "userId": 1
+}
+```
+
+**Response:** `201`
+
+```json
+[
+  {
+    "userId": 1,
+    "displayName": "displayName",
+    "email": "email",
+    "addedAt": "2025-01-01T00:00:00.000Z"
+  }
+]
+```
+
+**Errors:** `400` Invalid request (missing userId, owner as collaborator), `409` User is already a collaborator
+
+---
+
+### `DELETE /api/notes/{id}/collaborators`
+
+Remove a collaborator
+
+Owner can remove anyone. Collaborator can only remove self (leave). Use `userId=self` to leave.
+
+**Parameters:**
+
+| Name | In | Type | Required | Description |
+|------|----|------|----------|-------------|
+| `id` | path | string | yes |  |
+| `userId` | query |  | yes |  |
+
+**Response:** `200`
+
+```json
+{
+  "success": true
+}
+```
+
+---
+
+## Users
+
+### `GET /api/users/search`
+
+Search users
+
+Search registered users by email or display name. Excludes the current user. Any authenticated user can access.
+
+**Parameters:**
+
+| Name | In | Type | Required | Description |
+|------|----|------|----------|-------------|
+| `q` | query | string | yes | Search query (matches email or displayName) |
+
+**Response:** `200`
+
+```json
+[
+  {
+    "id": 1,
+    "displayName": "displayName",
+    "email": "email"
+  }
+]
+```
+
+---
+
 ## Attachments
 
 ### `GET /api/notes/{id}/attachments`

docs/ARCHITECTURE.md

@@ -98,8 +98,21 @@ Key tables:
 - `note_tags` - Many-to-many note ↔ tag
 - `attachments` - Image file metadata (path + thumbnailPath)
 - `api_keys` - MCP API keys (SHA-256 hashed, prefix for display)
+- `note_collaborators` - Sharing relationships (noteId + userId + addedBy)
+- `note_user_state` - Per-user pin/archive/sortOrder for shared notes
 - `sync_log` - Sync operation history
 
+### Collaboration Access Control
+
+Notes support sharing with other users on the same instance:
+
+- **Owner**: Full control (edit, trash, delete, share/unshare)
+- **Collaborator**: Can edit content, color, checklist mode; has independent pin/archive/sortOrder state via `note_user_state`
+- Access checks use `canAccessNote()` / `requireNoteAccess()` / `requireNoteOwnership()` helpers
+- `note_user_state` stores per-user view state (pinned, archived, sortOrder) — collaborators see their own organization
+- When owner trashes a note, it vanishes for all collaborators
+- Collaborators can leave (remove themselves) from shared notes
+
 ## MCP Server
 
 Crumbs includes a built-in [Model Context Protocol](https://modelcontextprotocol.io/) server that allows AI assistants (Claude Code, Claude Desktop, etc.) to interact with notes programmatically.

docs/FEATURES.md

@@ -80,6 +80,18 @@
 - Restore notes from trash
 - Permanently delete from trash
 
+### Note Sharing / Collaboration
+- Share notes with other users on the same instance
+- User search autocomplete for adding collaborators
+- Full collaboration: both owner and collaborators can edit content, color, and checklist mode
+- Per-user state: pin, archive, and sort order are independent per user
+- Owner-only actions: trash, delete, share/unshare
+- Collaborator indicator (Users icon) on shared note cards
+- Hover popover showing collaborator names
+- Share dialog accessible from editor toolbar
+- Collaborators can leave shared notes
+- Shared notes appear mixed in main list alongside owned notes
+
 ### Full-Text Search
 - Search across note titles, content, and tags
 - Real-time search results as you type

docs/openapi.yaml

@@ -168,6 +168,104 @@ paths:
         '404':
           description: Note not found
 
+  /api/notes/{id}/collaborators:
+    parameters:
+      - name: id
+        in: path
+        required: true
+        schema:
+          type: string
+          format: uuid
+    get:
+      summary: List collaborators
+      description: Returns all collaborators for a note. Requires access (owner or collaborator).
+      tags: [Collaboration]
+      responses:
+        '200':
+          description: Collaborator list
+          content:
+            application/json:
+              schema:
+                type: array
+                items:
+                  $ref: '#/components/schemas/Collaborator'
+    post:
+      summary: Add a collaborator
+      description: Owner-only. Adds a user as collaborator on the note.
+      tags: [Collaboration]
+      requestBody:
+        required: true
+        content:
+          application/json:
+            schema:
+              type: object
+              required: [userId]
+              properties:
+                userId:
+                  type: integer
+      responses:
+        '201':
+          description: Collaborator added, returns updated list
+          content:
+            application/json:
+              schema:
+                type: array
+                items:
+                  $ref: '#/components/schemas/Collaborator'
+        '400':
+          description: Invalid request (missing userId, owner as collaborator)
+        '409':
+          description: User is already a collaborator
+    delete:
+      summary: Remove a collaborator
+      description: Owner can remove anyone. Collaborator can only remove self (leave). Use `userId=self` to leave.
+      tags: [Collaboration]
+      parameters:
+        - name: userId
+          in: query
+          required: true
+          schema:
+            oneOf:
+              - type: integer
+              - type: string
+                enum: [self]
+      responses:
+        '200':
+          description: Collaborator removed
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/Success'
+
+  /api/users/search:
+    get:
+      summary: Search users
+      description: Search registered users by email or display name. Excludes the current user. Any authenticated user can access.
+      tags: [Users]
+      parameters:
+        - name: q
+          in: query
+          required: true
+          schema:
+            type: string
+          description: Search query (matches email or displayName)
+      responses:
+        '200':
+          description: Matching users (max 10)
+          content:
+            application/json:
+              schema:
+                type: array
+                items:
+                  type: object
+                  properties:
+                    id:
+                      type: integer
+                    displayName:
+                      type: string
+                    email:
+                      type: string
+
   /api/notes/{id}/attachments:
     parameters:
       - name: id
@@ -542,6 +640,19 @@ components:
         data:
           $ref: '#/components/schemas/NoteInput'
 
+    Collaborator:
+      type: object
+      properties:
+        userId:
+          type: integer
+        displayName:
+          type: string
+        email:
+          type: string
+        addedAt:
+          type: string
+          format: date-time
+
     ApiKey:
       type: object
       properties: