feat(ml): cadence-based video frame sampling + min-frame tag aggregation (#747)

Video tag noise root cause: frames were a FIXED count (6) max-pooled — a tag
firing on one frame survived at peak confidence, and a fixed count under-samples
long multi-scene videos so real scene-local tags looked like noise.

Redesign (operator-steered):
- Sample at a fixed CADENCE — one frame every `video_frame_interval_seconds`
  (default 4) across the 5–95% window — so a tag's frame-presence reflects real
  screen time independent of video length. Capped at `video_max_frames` (default
  64): a long video stretches the spacing instead of exploding into hundreds of
  inferences, bounding per-video cost on the single ml-worker (per-frame ffmpeg
  timeout also cut 60s→30s).
- Aggregate with `_aggregate_video_predictions`: keep a tag only if it appears in
  >= `video_min_tag_frames` sampled frames (≈ that many × interval seconds on
  screen — duration-independent noise rejection), with confidence = MEAN over the
  frames it appears in (not max). Clamps the threshold to the sample count so a
  1–2-frame short video still tags.
- All three knobs are DB-backed ml_settings (migration 0053), patchable via
  /api/ml/settings + sliders in the ML settings card — replaces the
  VIDEO_ML_FRAMES env var (product-not-project).

Tests: aggregation drops one-frame noise + means corroborated tags + clamps on
short videos; settings round-trip + min>max validation. Replaced the
_maxpool_predictions unit test.

NOTE: this is the QUALITY half of #747. The perf half — the ml-worker runs
CPU-only — is GPU enablement, tracked separately in #872.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/components/settings/MLThresholdSliders.vue

@@ -30,6 +30,39 @@
           </div>
         </v-col>
       </v-row>
+
+      <v-divider class="my-4" />
+
+      <div class="text-subtitle-2 mb-1">Video tagging</div>
+      <div class="text-caption fc-muted mb-3">
+        Videos are tagged by sampling frames at a fixed cadence. A tag is kept
+        only if it shows up in enough frames (≈ that many × the interval in
+        seconds of screen time), which filters one-frame noise without losing
+        tags that only appear in part of a longer video.
+      </div>
+      <v-row>
+        <v-col cols="12" sm="4">
+          <v-text-field
+            v-model.number="local.video_frame_interval_seconds"
+            label="Frame interval (s)" type="number" min="0.5" step="0.5"
+            density="comfortable" hide-details @change="save"
+          />
+        </v-col>
+        <v-col cols="12" sm="4">
+          <v-text-field
+            v-model.number="local.video_max_frames"
+            label="Max frames" type="number" min="1" step="1"
+            density="comfortable" hide-details @change="save"
+          />
+        </v-col>
+        <v-col cols="12" sm="4">
+          <v-text-field
+            v-model.number="local.video_min_tag_frames"
+            label="Min frames per tag" type="number" min="1" step="1"
+            density="comfortable" hide-details @change="save"
+          />
+        </v-col>
+      </v-row>
     </v-card-text>
     <v-card-text v-else><v-skeleton-loader type="paragraph" /></v-card-text>
   </v-card>
@@ -59,9 +92,14 @@ async function save() {
   const floor = local.tagger_store_floor
   local.suggestion_threshold_character = Math.max(local.suggestion_threshold_character, floor)
   local.suggestion_threshold_general = Math.max(local.suggestion_threshold_general, floor)
+  // Mirror the server invariant: a tag can't require more frames than are sampled.
+  local.video_min_tag_frames = Math.min(local.video_min_tag_frames, local.video_max_frames)
   const patch = {}
   for (const f of fields) patch[f.key] = local[f.key]
   patch.tagger_store_floor = local.tagger_store_floor
+  patch.video_frame_interval_seconds = local.video_frame_interval_seconds
+  patch.video_max_frames = local.video_max_frames
+  patch.video_min_tag_frames = local.video_min_tag_frames
   try { await store.patchSettings(patch) }
   catch (e) { toast({ text: e.message, type: 'error' }) }
 }