Fix attachment download error mapping and Content-Type resolution; add GC endpoint

[cuibonobo]

Summary

Addresses the five attachment-related issues (#52, #37, #46, #48, #38):

• POST /attachments must store bytes and create the _attachment@1 record atomically, returning the record #52 (upload atomicity) — already closed, no action.
• Attachment upload buffers the entire body in memory before the size check #46 (streaming body-limit) — already fixed in code (folded into the POST /attachments must store bytes and create the _attachment@1 record atomically, returning the record #52 commit); closing separately with an explanatory comment, no code change here.
• Fixes GET /attachments/:fileId error mapping: authenticated denials return 401, and every non-permission error collapses to 404 #48 — GET /attachments/:fileId no longer collapses every denial to 401 or masks unexpected errors as a bare 404. Only an anonymous StackPermissionError becomes 401; a verified-but-denied requester now gets 403; StackNotFoundError still maps to 404; anything else propagates to the global error handler (500, logged). Missing and inaccessible files stay indistinguishable for non-owners — core already guarantees this (ScopedStack.getAttachment throws the same error either way), the route just needed to stop overriding it.
• Fixes Core sync: attachment Content-Type — three-source resolution, stored mimeType, safe-list forcing (core #65, #66) #37 — the download route now resolves Content-Type from all three spec'd sources instead of two: ?contentType, then ?filename extension inference, then (newly implemented) the fileId's first-recorded _attachment@1 metadata record. Wired through core's resolveAttachmentDownloadContentType() (@haverstack/core/wire), replacing the local deny-list/extension-table with core's safe-list. Content-Disposition's filename now prefers the requester's own record, falling back to the first-recorded one.
• Fixes Core sync: orphaned-blob GC endpoint — owner-only POST /attachments/gc (core #64) #38 — adds owner-only POST /attachments/gc, wired to ScopedStack.collectAttachmentGarbage(), accepting optional { graceMs, dryRun } and returning { deleted, reclaimedBytes }.

Incidental dependency fix

Bumps @haverstack/adapter-local to 0.9.0. While fixing #48 I removed a blanket catch that had been silently absorbing every non-permission error as a hardcoded 404 — with it gone, a genuinely-missing attachment started 500ing instead of 404ing. Root cause: the resolved @haverstack/blob-adapter-disk@0.7.0 (transitive via adapter-local@0.8.0) still depended on @haverstack/core@^0.9.0 while the rest of the tree was on 0.10.0, so DiskBlobAdapter's StackNotFoundError was a different class than the one @haverstack/wire-types' serializeError checks for, and instanceof silently failed.

This went through two states: an earlier commit on this branch pinned blob-adapter-disk to ^0.8.0 via a pnpm.overrides workaround, since adapter-local@0.8.0's own declared range (^0.7.0) excluded the fixed version. adapter-local@0.9.0 has since been published upstream with the corrected blob-adapter-disk: ^0.8.0 dependency, so this PR now just bumps the real dependency and the override is gone.

Test plan

• pnpm typecheck clean
• pnpm lint clean
• pnpm format:check clean
• pnpm test — 238/238 passing, including new coverage:
  • GET /attachments/:fileId error mapping: authenticated denials return 401, and every non-permission error collapses to 404 #48: authenticated-denial tests updated from 401→403; added an explicit anti-oracle pin (missing vs. inaccessible file → identical response for a non-owner)
  • Core sync: attachment Content-Type — three-source resolution, stored mimeType, safe-list forcing (core #65, #66) #37: stored-metadata Content-Type resolution, first-recorded-vs-requester's-own filename precedence, dangerous stored-type forcing, nosniff-always
  • Core sync: orphaned-blob GC endpoint — owner-only POST /attachments/gc (core #64) #38: anonymous→401, non-owner→403, empty body, dry-run leaves the file intact, grace period honored, past-grace deletion reclaims bytes, referenced files are skipped
• docs/api.md updated with the GC endpoint and the download resolution/anti-oracle behavior

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https://claude.ai/code/session_012GdVbbu2KmZkfcvRmY4BRq