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/auth.py

@@ -359,7 +359,10 @@ async def oauth_callback():
         return redirect("/login?error=oauth")
 
     sub = claims.get("sub", "")
-    email = claims.get("email", "")
+    # Only trust the email claim for account linking if the provider has verified it.
+    # An unverified email could be used to hijack an existing local account.
+    email_verified = claims.get("email_verified", False)
+    email = claims.get("email", "") if email_verified else ""
     preferred_username = claims.get("preferred_username", "")
 
     if not sub: