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/utils/safe_probe.py

@@ -60,6 +60,9 @@ class ProbeResult:
     reason: str | None = None
     width: int | None = None
     height: int | None = None
+    # Container duration in seconds (videos only) — the Tier-1 video near-dup
+    # key (#871). None when not a video / ffprobe didn't report it.
+    duration: float | None = None
 
 
 def probe_video(path: Path, *, timeout: float = VIDEO_PROBE_TIMEOUT_SECONDS) -> ProbeResult:
@@ -69,7 +72,10 @@ def probe_video(path: Path, *, timeout: float = VIDEO_PROBE_TIMEOUT_SECONDS) ->
             [
                 "ffprobe", "-v", "error",
                 "-select_streams", "v:0",
-                "-show_entries", "stream=width,height",
+                # format=duration is the reliable container duration (stream
+                # duration is often N/A for remuxed/streamed mp4) — the #871
+                # video near-dup key.
+                "-show_entries", "stream=width,height:format=duration",
                 "-of", "json", str(path),
             ],
             capture_output=True, text=True, timeout=timeout,
@@ -87,13 +93,21 @@ def probe_video(path: Path, *, timeout: float = VIDEO_PROBE_TIMEOUT_SECONDS) ->
             reason=f"ffprobe rejected the file: {out.stderr.strip()[:200]}",
         )
     try:
-        streams = (json.loads(out.stdout) or {}).get("streams") or []
+        parsed = json.loads(out.stdout) or {}
     except json.JSONDecodeError as exc:
         return ProbeResult(ok=False, crashed=False, reason=f"ffprobe output parse failed: {exc}")
+    streams = parsed.get("streams") or []
     if not streams:
         return ProbeResult(ok=False, crashed=False, reason="no decodable video stream")
+    duration = None
+    raw_dur = (parsed.get("format") or {}).get("duration")
+    try:
+        duration = float(raw_dur) if raw_dur is not None else None
+    except (TypeError, ValueError):
+        duration = None
     return ProbeResult(
         ok=True, width=streams[0].get("width"), height=streams[0].get("height"),
+        duration=duration,
     )