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>

backend/app/models/image_record.py

@@ -13,6 +13,7 @@ from sqlalchemy import (
     BigInteger,
     DateTime,
     Enum,
+    Float,
     ForeignKey,
     Integer,
     String,
@@ -39,6 +40,10 @@ class ImageRecord(Base):
     mime: Mapped[str] = mapped_column(String(64), nullable=False)
     width: Mapped[int | None] = mapped_column(Integer, nullable=True)
     height: Mapped[int | None] = mapped_column(Integer, nullable=True)
+    # Video container duration (seconds); NULL for images. The Tier-1 video
+    # near-dup key (#871): two videos of the same artist with matching duration
+    # (+ aspect) are the same content across re-encodes — dedup like image pHash.
+    duration_seconds: Mapped[float | None] = mapped_column(Float, nullable=True)
 
     # Integrity verification status. FC-2e populates this; FC-2a leaves rows at 'unknown'.
     # Values: 'unknown' (default), 'ok', 'corrupt', 'failed_verification'.