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>

apps/cli/src/cli.ts

@@ -12,6 +12,7 @@ import {
   cmdObjectsDownload,
   cmdObjectsList,
   cmdObjectsRemove,
+  cmdObjectsPresign,
   cmdObjectsUpload,
 } from "./commands/objects.js";
 import { cmdOrgsCreate, cmdOrgsList } from "./commands/orgs.js";
@@ -114,6 +115,12 @@ cli
   .command("objects:rm <bucket> <key>", "Delete an object")
   .option("--project <id>", "Project override")
   .action(cmdObjectsRemove);
+cli
+  .command("objects:presign <bucket> <key>", "Mint a presigned URL (s3 driver)")
+  .option("--project <id>", "Project override")
+  .option("--upload", "PUT URL for direct upload instead of GET download")
+  .option("--ttl <seconds>", "Lifetime 60-3600 (default 900)")
+  .action(cmdObjectsPresign);
 
 cli
   .command("keys:list", "List API keys for the current project")