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/services/rss.py

@@ -55,6 +55,10 @@ async def fetch_and_cache_feed(feed_id: int, url: str) -> int:
     Fetch a feed URL, parse it, and upsert new items into rss_items.
     Returns the number of new items stored.
     """
+    scheme = url.split("://")[0].lower() if "://" in url else ""
+    if scheme not in ("http", "https"):
+        logger.warning("Blocked RSS fetch with non-http(s) scheme: %s", url[:80])
+        return 0
     try:
         parsed = await _parse_feed(url)
     except Exception: