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>

alembic/versions/0053_ml_settings_video_tagging.py

@@ -0,0 +1,49 @@
+"""ml_settings: video tagging knobs (cadence sampling + noise floor)
+
+#747. Video tag quality/perf: sample frames at a fixed cadence (interval) so a
+tag's frame-presence reflects real screen time, cap total frames so long videos
+stay bounded, and keep a tag only if it appears in >= min_tag_frames sampled
+frames. Operator-tunable via Settings → ML (replaces the VIDEO_ML_FRAMES env var).
+
+Revision ID: 0053
+Revises: 0052
+Create Date: 2026-06-16
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0053"
+down_revision: Union[str, None] = "0052"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "ml_settings",
+        sa.Column(
+            "video_frame_interval_seconds", sa.Float(), nullable=False,
+            server_default="4.0",
+        ),
+    )
+    op.add_column(
+        "ml_settings",
+        sa.Column(
+            "video_max_frames", sa.Integer(), nullable=False, server_default="64",
+        ),
+    )
+    op.add_column(
+        "ml_settings",
+        sa.Column(
+            "video_min_tag_frames", sa.Integer(), nullable=False,
+            server_default="3",
+        ),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("ml_settings", "video_min_tag_frames")
+    op.drop_column("ml_settings", "video_max_frames")
+    op.drop_column("ml_settings", "video_frame_interval_seconds")