buyer: response archive (SQLite + filesystem) for off-chain audit trail

The on-chain Submit tx anchors sha256(canonicalize(receipt)) so the
receipt's signature is provably tied to a specific Cardano block — but
without persisting the receipt content + the actual response bytes
nowhere can buyer or supplier produce the artefacts a dispute would
need to argue about. This commit closes that gap on the buyer side.

What's new

- buyer/src/db/schema.ts (new):
  `responses` table keyed by escrow_ref, indexes on completed_at /
  capability_id / supplier_pkh. Stores metadata + receipt JSON +
  Ed25519 signature; large blobs (request envelope, response bytes)
  live on disk per-escrow.

- buyer/src/db/archive.ts (new):
  ResponseArchive class — persistChat / persistTts / list / get /
  readRequest / readResponse. Filesystem layout under ARCHIVE_DIR:
    <txhash>_<index>/request.json   (always JSON)
    <txhash>_<index>/response.{ext} (txt/json for chat, mp3/wav/… for TTS)
  Synchronous (better-sqlite3 + fs.writeFileSync) — volume is low and
  the simplicity is worth more than async overhead.

- buyer/src/server.ts:
  - AppDeps.archive plumbed through createApp.
  - /v1/submit-prompt + /v1/submit-tts persist BEFORE returning to
    the SPA, using canonicalize() for chat (same bytes the supplier
    hashed; JSON.stringify produced different bytes → wrong sha256).
    Persist failures are logged but never abort the response — the
    on-chain receipt is still authoritative.
  - New read-only endpoints:
      GET /v1/responses?limit=…           list metadata + parsed receipts
      GET /v1/responses/:ref               single-row detail
      GET /v1/responses/:ref/request       request artefact (application/json)
      GET /v1/responses/:ref/response      response artefact (verbatim bytes)
  - URL path uses "<txhash>_<index>" (the dir-safe form) since "#"
    can't appear unencoded in URLs (fragment delimiter); server
    normalises back to canonical "<txhash>#<index>" for SQLite lookup.
  - 503 service_unavailable on /v1/responses* when boot has no
    archive (ARCHIVE_DIR unset or DB unopenable) — the rest of
    the buyer-app still works.

- buyer/src/ui/pages/TaskHistory.tsx:
  Joins indexer escrows (chain state) + /v1/responses (archived
  artefacts) by escrow_ref. Each completed lifecycle gets an
  archive panel showing:
  - Request panel (above, chronological order): human-readable
    summary — first user message for chat, `text + voice + format +
    speed` for TTS, raw JSON fallback otherwise.
  - Response panel: assistant text inline for chat, <audio controls>
    for TTS; Download response / Download request anchors.
  - "Verify hash" button — fetches the response bytes, computes
    sha256 via crypto.subtle, compares to receipt.response_hash;
    green ✓ on match, red ✗ with truncated computed/expected on
    mismatch.
  - Collapsible "Receipt + signature" with the full canonical
    receipt JSON + the supplier's Ed25519 hex sig.

- buyer/src/config.ts: + ARCHIVE_DIR env (default "./data/archive").

- buyer/Dockerfile: builder gains python3/make/g++ + the
  better-sqlite3 native-binding fetch step (mirrors indexer/Dockerfile);
  runtime image stays toolchain-free.

- deploy/testnet/docker-compose.buyer.yml: bind-mounts
  ../../data/buyer/archive (repo-root-relative, not file-relative)
  into /data/archive. ARCHIVE_DIR env points at that path.

Tests

- tests/unit/buyer-archive.test.ts (new, 14 tests): persistChat /
  persistTts unit coverage (filesystem layout, INSERT-OR-REPLACE
  semantics, content-type → extension mapping for mp3/wav/opus/aac/
  flac, list ordering + limit, single-row get/read miss-vs-hit) +
  HTTP integration (503 when disabled, happy paths returning
  metadata / request / response artefact bytes with correct
  content-types, 404 on unknown ref).
- tests/unit/buyer-ui-smoke.test.tsx: TaskHistory fetch mock now
  also handles /v1/responses (returns empty list — same code path as
  "archive disabled" production case).

45 buyer tests passing; chat + TTS lifecycle verified end-to-end on
Vector testnet; "Verify hash" button confirms sha256(response bytes)
matches receipt.response_hash for both capabilities.

deploy/testnet/docker-compose.buyer.yml

@@ -40,6 +40,17 @@ services:
       # apex-simulation). When this is unset, /v1/synth-speech responds 503
       # and the SPA's PiperTTSForm shows a clear error.
       TTS_PIPER_BASE_URL: "https://tts-piper.vector.testnet.apexfusion.org"
+      # Response archive — SQLite metadata + per-escrow artefact files for
+      # off-chain dispute resolution. Host-bind-mounted below so it survives
+      # container rebuilds and an operator can `ls`/back-up directly.
+      ARCHIVE_DIR: /data/archive
+    volumes:
+      # Bind-mount path is repo-root-relative (../../) so the on-disk
+      # archive lives at $REPO_ROOT/data/buyer/archive — same convention
+      # we use elsewhere. Without this, docker-compose's default would
+      # place the volume at deploy/testnet/data/... (relative to THIS
+      # file), which is harder to find and back up.
+      - ../../data/buyer/archive:/data/archive
     healthcheck:
       test: ["CMD-SHELL", "wget -qO- http://localhost:8070/healthz >/dev/null || exit 1"]
       interval: 20s