security: fix 10 vulnerabilities from security audit

- SSRF: block private/internal URLs in image cache fetch
- SSRF: block private/internal URLs in RSS feed fetch (scheme guard)
- SSRF: block private/internal URLs in CalDAV URL setting
- Auth: require login for GET /api/images/<id> (was unauthenticated)
- Auth: restrict Ollama model pull/delete to admin users only
- Info disclosure: remove email from /api/users/search response
- OAuth: skip email-based account linking when email_verified is false
- Config: raise hard error on default SECRET_KEY when SECURE_COOKIES=true
- Rate limit: document proxy header requirement; add startup warning
- XSS: remove src/alt from global DOMPurify ADD_ATTR allowlist

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

src/fabledassistant/routes/users.py

@@ -24,6 +24,6 @@ async def search_users():
             ).limit(10)
         )).scalars().all()
     return jsonify({"users": [
-        {"id": u.id, "username": u.username, "email": u.email}
+        {"id": u.id, "username": u.username}
         for u in users
     ]})