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/config.py

@@ -88,5 +88,10 @@ class Config:
             errors.append(f"SMTP_PORT={cls.SMTP_PORT} must be between 1 and 65535")
         if cls.oidc_enabled() and not cls.BASE_URL.startswith(("http://", "https://")):
             errors.append(f"BASE_URL='{cls.BASE_URL}' must start with http:// or https:// when OIDC is enabled")
+        if cls.SECRET_KEY == "dev-secret-change-me" and cls.SECURE_COOKIES:
+            errors.append(
+                "SECRET_KEY is set to the insecure default but SECURE_COOKIES=true indicates "
+                "a production deployment. Set SECRET_KEY or SECRET_KEY_FILE before starting."
+            )
         if errors:
             raise ValueError("Configuration errors:\n" + "\n".join(f"  - {e}" for e in errors))