fix: implement fixable limitations from annotation pass

cache.py — model fingerprint auto-invalidation:
  Replace hardcoded "buffalo_l_v1" version string with a fingerprint
  derived from buffalo_l .onnx file sizes and mtimes. EmbeddingCache now
  computes this at init time; stale embeddings from replaced or updated
  model files are automatically invalidated. Falls back to the static
  string before the model is downloaded.
  Note: existing caches built against the old key will miss on the first
  run after upgrade and recompute cleanly.

frigate_api.py — Frigate version check:
  Add get_frigate_version() (GET /api/version). Called at the start of
  upload_to_frigate(); warns if below v0.16 where the face training API
  endpoints don't exist.

immich_api.py + cli.py — Immich version check:
  Add get_immich_version() (GET /api/server/version). Called at startup
  before get_people(); warns if below v1.106 where the face data and
  merge APIs winnow depends on aren't guaranteed present.

Remaining TODO(frigate-api) annotations are left in place — they require
Frigate to expose per-file embeddings or a rebuild-complete signal before
they can be addressed.

winnow/frigate_api.py

@@ -8,6 +8,24 @@ import requests
 logger = logging.getLogger(__name__)
 
 
+def get_frigate_version() -> str | None:
+    """Fetch Frigate's version string from GET /api/version.
+
+    Returns the version string (e.g. "0.16.0-beta4") or None if FRIGATE_URL
+    is unset, the endpoint is unreachable, or the response is not parseable.
+    """
+    frigate_url = os.environ.get("FRIGATE_URL", "").rstrip("/")
+    if not frigate_url:
+        return None
+    try:
+        resp = requests.get(f"{frigate_url}/api/version", timeout=5)
+        if resp.ok:
+            return resp.text.strip().strip('"')
+        return None
+    except Exception:
+        return None
+
+
 def _get_faces_data() -> dict | None:
     """Fetch raw GET /api/faces response. Returns None if unavailable."""
     frigate_url = os.environ.get("FRIGATE_URL", "").rstrip("/")