Withdraw the file-hash tier; document the legal posture

Removes `cut.video_hash` and the `exact` match tier on legal grounds. The
OpenSubtitles hash was the strongest technical signal available — it identifies
a specific file, so it cannot produce a false positive — and that is exactly
the problem.

Every tier must be a claim about a *cut*, never about a copy. A TMDB id
discloses "some copy of this film", which is what a library catalogue
discloses. A file hash discloses "this exact release": it made a read endpoint
into a release-level oracle, and made an instance's database a mapping from
file fingerprints to the instances holding them. That is a far more specific
disclosure than PR-005 permits, and a dataset no volunteer operator should be
asked to hold. The audio signature is the replacement: derived from content, it
identifies the cut rather than the copy, so two encodes of the same edit agree.

The field is deleted rather than kept as a vestigial null, on the same
reasoning §2 applied to `anneal_sec` — a key naming a signal the format no
longer has is actively misleading — so an upload carrying one is now an
unknown-field 400, with a test asserting it.

**Every content_id changes**, including for manifests that never carried a
hash, because the canonical `cut` object lost a key. The golden vector is
regenerated and re-verified against an independent Python implementation; the
plugin and extraction repos must adopt the new value or federation
deduplication silently breaks. Free now, pre-release; not free later.

Adds docs/legal-posture.md, the operator-facing half of what §5a asks for:
what an instance holds exhaustively, what it structurally cannot do, and how
that sits against the intermediary-liability regimes that plausibly apply.

208 tests. Coverage 25/32.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

TRACES: UR-011 | SR-004, PR-005

tests/api.rs

@@ -154,7 +154,7 @@ fn movie_manifest(tmdb_id: &str, runtime: f64) -> Value {
         "jmanifest_version": 2,
         "identity": { "type": "movie", "tmdb_id": tmdb_id, "title": "The Death of Stalin",
                       "year": 2017 },
-        "cut": { "runtime_sec": runtime, "video_hash": "opensubtitles:8e245d9679d31e12" },
+        "cut": { "runtime_sec": runtime },
         "extraction": { "sample_fps": 5, "extinction_sec": 12,
                         "pipeline_version": "scene-actor-extraction 0.4.1",
                         "gallery_scope": "global" },
@@ -405,7 +405,7 @@ async fn missing_runtime_is_rejected() {
     let s = TestServer::new("no-runtime");
     let token = s.token().await;
     let mut m = movie_manifest("504172", 6420.5);
-    m["cut"] = json!({ "video_hash": "opensubtitles:8e245d9679d31e12" });
+    m["cut"] = json!({ "container_duration_sec": 6420.5 });
     let (status, _, _) = s.post_json_auth("/api/v1/manifests", &token, &m).await;
     assert_eq!(status, StatusCode::BAD_REQUEST);
 }