category_fetcher: persist batch API probe result across sessions

The probe that detects whether a site's batch tag API works
(Gelbooru proper: yes, Rule34: no) now persists its result in the
tag_types table using a sentinel key (__batch_api_probe__). On
subsequent app launches, the fetcher reads the saved result at
construction time and skips the probe entirely.

Before: every session with Rule34 wasted ~0.6s on a probe request
that always fails (Rule34 returns garbage for names=). During that
time the background prefetch couldn't start HTML scraping, so the
first few post clicks paid ~0.3s each.

After: first ever session probes Rule34 once, stores False. Every
subsequent session reads False from DB, skips the probe, and the
background prefetch immediately starts HTML scraping. By the time
the user clicks any post, the scrape is usually done.

Gelbooru proper: probe succeeds on first session, stores True.
Future sessions use the batch API without probing. No change in
speed (already fast), just saves the probe roundtrip.

Persisted per site_id so different Gelbooru-shaped sites get their
own probe result. The clear_tag_cache method wipes probe results
along with tag data (the sentinel key lives in the same table).

booru_viewer/core/api/category_fetcher.py

@@ -127,24 +127,36 @@ class CategoryFetcher:
         self._sem = asyncio.Semaphore(self._PREFETCH_CONCURRENCY)
         self._inflight: dict[int, asyncio.Task] = {}
 
-        self._batch_api_works: bool | None = None
-        # Probe state for the batch tag API:
+        # Probe state for the batch tag API. Persisted to DB so
+        # the probe runs at most ONCE per site, ever. Rule34's
+        # broken batch API is detected on the first session; every
+        # subsequent session skips the probe and goes straight to
+        # HTML prefetch (saving ~0.6s of wasted probe time).
         #
         #   None  — not yet probed, OR last probe hit a transient
-        #           error (HTTP error, timeout, parse exception).
-        #           Next prefetch_batch will retry the probe.
-        #   True  — probe succeeded (response contained >=1 of the
-        #           requested names). Batch API used for all future
-        #           calls on this instance.
-        #   False — probe got a clean HTTP 200 with zero matching
-        #           names for ANY of the requested tags.  The API
-        #           is structurally broken on this site (Rule34's
-        #           ``names=`` filter returns unrelated tags).
-        #           Per-post HTML used for all future calls.
-        #
-        # Transition to False is permanent for the instance lifetime.
-        # Transition to True is permanent for the instance lifetime.
-        # None -> None on transient error preserves retry ability.
+        #           error. Next prefetch_batch retries the probe.
+        #   True  — probe succeeded (Gelbooru proper). Permanent.
+        #   False — clean 200 + zero matching names (Rule34).
+        #           Permanent. Per-post HTML from now on.
+        self._batch_api_works = self._load_probe_result()
+
+    # ----- probe result persistence -----
+
+    _PROBE_KEY = "__batch_api_probe__"  # sentinel name in tag_types
+
+    def _load_probe_result(self) -> bool | None:
+        """Read the persisted probe result from the DB, or None."""
+        row = self._db.get_tag_labels(self._site_id, [self._PROBE_KEY])
+        val = row.get(self._PROBE_KEY)
+        if val == "true":
+            return True
+        elif val == "false":
+            return False
+        return None
+
+    def _save_probe_result(self, result: bool) -> None:
+        """Persist the probe result so future sessions skip the probe."""
+        self._db.set_tag_labels(self._site_id, {self._PROBE_KEY: "true" if result else "false"})
 
     # ----- cache compose (instant, no HTTP) -----
 
@@ -421,8 +433,10 @@ class CategoryFetcher:
         cached = self._db.get_tag_labels(self._site_id, list(all_tags))
         missing = [t for t in all_tags if t not in cached]
         if not missing:
-            # Everything's cached — can't probe, assume batch works
-            self._batch_api_works = True
+            # Everything's cached — can't probe, skip
+            if self._batch_api_works is None:
+                self._batch_api_works = True
+                self._save_probe_result(True)
             for p in posts:
                 self.try_compose_from_cache(p)
             return True
@@ -477,6 +491,7 @@ class CategoryFetcher:
 
         if got_any:
             self._batch_api_works = True
+            self._save_probe_result(True)
             if matched:
                 self._db.set_tag_labels(self._site_id, matched)
             # Fetch any remaining missing tags via the batch path
@@ -485,6 +500,7 @@ class CategoryFetcher:
         else:
             # Clean 200 but zero matching names → structurally broken
             self._batch_api_works = False
+            self._save_probe_result(False)
             return False