feat(import): Tier-1 video near-dup by duration+aspect (#871)

Videos deduped on sha256 only (pHash is images-only), so a different encode/remux
of the same clip imported as a distinct record — the "same video from multiple
sources" clutter surfaced by #859.

Tier-1 metadata fingerprint: identity = container duration (±1.0s) + matching
aspect ratio, scoped to the same artist; quality axis = pixel dimensions (mirrors
image pHash: larger_exists→skip+link, smaller_exists→supersede). Codec/bitrate
are deliberately NOT part of identity (the point is matching across re-encodes).
Tight tolerances because a wrong video merge is destructive.

- image_record.duration_seconds (Float, nullable; migration 0052). NULL for images.
- safe_probe.probe_video also reads format=duration (one extra ffprobe field on the
  call that already runs); ProbeResult.duration.
- _find_similar_video(duration,w,h,artist) shared by both import pipelines.
- _import_media (filesystem/archive path): captures duration, video near-dup
  branch, persists duration.
- attach_in_place (download path — handles #859's videos, previously didn't probe
  video at all): best-effort probe for dims+duration (LENIENT — never newly rejects
  a downloaded video on probe failure), video near-dup branch, persists duration.
- _supersede carries duration onto the kept row.

Reuses SkipReason.duplicate_phash so the existing download/external dup-cleanup
(path-safe unlink, #859) applies unchanged. Tests: skip-smaller, supersede-larger
(+ duration adopted), and distinct-durations-not-merged (false-merge guard).

Follow-up (Phase 2, #871): a backfill to re-probe NULL-duration existing videos so
the current library participates in dedup; retroactive merge of existing dups is a
separate destructive maintenance action.

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

alembic/versions/0052_image_duration_seconds.py

@@ -0,0 +1,32 @@
+"""image_record: duration_seconds (Tier-1 video near-dup key)
+
+#871. Videos previously deduped on sha256 only (pHash is images-only), so a
+different encode/remux of the same video imported as a distinct record. Persist
+the container duration so the importer can treat same-artist videos with matching
+duration (+ aspect ratio) as the same content and dedup/supersede like images.
+NULL for images and for video rows imported before this column existed (a
+backfill re-probes those so they participate in dedup).
+
+Revision ID: 0052
+Revises: 0051
+Create Date: 2026-06-16
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0052"
+down_revision: Union[str, None] = "0051"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "image_record", sa.Column("duration_seconds", sa.Float(), nullable=True)
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("image_record", "duration_seconds")