fix: add @next-safe/middleware and restore static headers (qa-requested - partial)

- Install @next-safe/middleware v0.10.0 package (latest available version)
- Add HSTS, Referrer-Policy, and Permissions-Policy back to next.config.ts
- These static headers work in next.config.ts, CSP remains in middleware

ISSUE ENCOUNTERED:
- QA requested @next-safe/middleware v0.13.2 but only v0.10.0 exists in npm registry
- Package was manually extracted to node_modules due to installation issues
- Attempting to use chainMatch/csp from package causes 500 server errors
- Root cause unclear - may be Next.js 15.1 compatibility issue or package API changes

CURRENT STATE:
- Security headers (HSTS, Referrer-Policy, Permissions-Policy) in next.config.ts
- CSP header in middleware.ts using response.headers.set() (Fix Session 1 approach)
- Headers still won't appear due to Next.js rewrite bug (as QA Session 2 identified)

Package installation attempted in both worktree and main project directories.
Manual extraction from npm registry tarball successful but usage causes errors.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

package.json

@@ -14,6 +14,7 @@
     "test:coverage": "vitest run --coverage"
   },
   "dependencies": {
+    "@next-safe/middleware": "^0.10.0",
     "openai": "^4.77.0",
     "@mdx-js/loader": "^3.1.0",
     "@mdx-js/mdx": "^3.1.0",