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

src/api/exists.rs

@@ -128,12 +128,7 @@ async fn lookup_one(state: &AppState, params: &LookupParams) -> ApiResult<Exists
 
             let cuts: Vec<(String, StoredCut)> = candidates
                 .iter()
-                .map(|m| {
-                    (
-                        m.id.clone(),
-                        StoredCut { runtime_sec: m.runtime_sec, video_hash: m.video_hash.clone() },
-                    )
-                })
+                .map(|m| (m.id.clone(), StoredCut { runtime_sec: m.runtime_sec }))
                 .collect();
 
             let Some((id, m)) = matching::best_match(&client_cut, &cuts) else {