feat: Presigned object URLs (s3 driver)

POST /buckets/:id/object-presigns mints SigV4 query-signed GET/PUT
URLs so clients move bytes directly against S3 — large transfers skip
the backend entirely.

- storage.Presigner optional interface; S3Store implements, local
  driver returns a clear error (no external surface to sign).
- TTL 60s–1h (default 15m); download requires read role + existing
  object row (no key probing); upload requires write role.
- S3_PUBLIC_ENDPOINT rewrites the client-facing host for internal
  S3_ENDPOINTs — signatures bind the host, URLs are valid only where
  minted.
- CLI: primora objects:presign <bucket> <key> [--upload] [--ttl].

Verified live against cloudserver: GET fetched the object with no
auth headers; PUT pushed 1 MB direct to S3; unsigned list → 403;
untracked-key download presign → 404.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

QUICK_START.md

@@ -67,6 +67,7 @@ The important ones:
 | `USER_RATE_LIMIT_PER_MINUTE` / `API_KEY_RATE_LIMIT_PER_MINUTE` | Throttling defaults — overridable in-app |
 | `BACKEND_STORAGE_DRIVER` | `local` (default, `BACKEND_STORAGE_ROOT`) or `s3` — S3-compatible object storage |
 | `S3_ENDPOINT` / `S3_BUCKET` / `S3_ACCESS_KEY_ID` / `S3_SECRET_ACCESS_KEY` | S3 backend config (with `S3_REGION`, `S3_PREFIX`, `S3_PATH_STYLE`) |
+| `S3_PUBLIC_ENDPOINT` | Client-facing S3 host for presigned URLs when `S3_ENDPOINT` is internal-only |
 | `FUNCTIONS_RUNTIME` / `FUNCTIONS_TIMEOUT_SECONDS` | JS runtime for Functions — `auto` (default, resolves `bun`/`deno` from PATH) or a binary path; 30 s default timeout |
 | `VITE_DEMO_MODE` | Enable the client-side demo workspace |