docs: add open source community files and README badges

Add LICENSE, CONTRIBUTING, CHANGELOG, SECURITY, CODE_OF_CONDUCT,
GitHub issue/PR templates, CODEOWNERS. Update package.json with
license/repo/author metadata. Add badges and logo to README.

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

SECURITY.md

@@ -0,0 +1,44 @@
+# Security Policy
+
+## Supported Versions
+
+| Version | Supported          |
+|---------|--------------------|
+| latest  | Yes                |
+| < latest | No                |
+
+We recommend always running the latest version of Crumbs.
+
+## Reporting a Vulnerability
+
+**Please do not report security vulnerabilities through public GitHub issues.**
+
+Instead, please report them via [GitHub's private vulnerability reporting](https://github.com/bretzel-app/crumbs/security/advisories/new).
+
+Include as much of the following as possible:
+
+- Description of the vulnerability
+- Steps to reproduce
+- Impact assessment
+- Suggested fix (if any)
+
+You should receive an initial response within 48 hours. We will work with you to understand the issue and coordinate a fix before any public disclosure.
+
+## Scope
+
+Since Crumbs is a self-hosted application, the following are in scope:
+
+- Authentication bypass or session hijacking
+- SQL injection or data leaks
+- Cross-site scripting (XSS)
+- Server-side request forgery (SSRF)
+- Path traversal or file access vulnerabilities
+- Denial of service via crafted input
+
+## Disclosure Policy
+
+We follow coordinated disclosure. Once a fix is available, we will:
+
+1. Release a patched version
+2. Publish a security advisory on GitHub
+3. Credit the reporter (unless anonymity is requested)