feat: add image attachment backend — batch fetch, API responses, optimization, thumbnails

- Add fetchAttachmentsForNotes() batch helper mirroring fetchTagsForNotes pattern
- Include attachments in all note API responses (list, single, sync)
- Fix ImageUpload.svelte URL bug (path-based → query param)
- Add client-side image optimization (resize, compress, WebP conversion)
- Add thumbnailPath column to attachments schema with migration
- Update saveAttachment/deleteAttachment for thumbnail file handling
- Update attachment API: accept thumbnail in POST, serve via &thumb=1 in GET
- Add Cache-Control immutable header for served attachments

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

drizzle/meta/_journal.json

@@ -0,0 +1,13 @@
+{
+  "version": "7",
+  "dialect": "sqlite",
+  "entries": [
+    {
+      "idx": 0,
+      "version": "6",
+      "when": 1773000424830,
+      "tag": "0000_wooden_vargas",
+      "breakpoints": true
+    }
+  ]
+}