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>

.env.example

@@ -60,6 +60,10 @@ BACKEND_STORAGE_DRIVER=local
 #S3_SECRET_ACCESS_KEY=
 #S3_PREFIX=
 #S3_PATH_STYLE=true
+# Client-facing S3 endpoint for presigned URLs — set when S3_ENDPOINT is an
+# internal address (e.g. http://minio:9000 inside compose) that browsers
+# can't reach. Presigned URLs are valid only at the host they were minted for.
+#S3_PUBLIC_ENDPOINT=https://objects.example.com
 
 # Mail defaults — each is overridable in-app (Settings → Instance). In-app
 # values win over these env vars; deleting the in-app value reverts here.