StorageAdapter: normalize not-found errors across adapters

[arndvs]

Context

PR #51 review (Round 3, thread #7) flagged that readJson() in brand-loader.ts catches isENOENT(err) to map missing files to BrandIncompleteError. The logo route (app/api/brands/[slug]/logos/[id]/route.ts) has the same pattern.

Currently only LocalFsAdapter is implemented, so isENOENT() (checking err.code === "ENOENT") works. When AzureBlobAdapter lands (Slice 4), Azure SDK throws RestError with statusCode === 404 / code === "BlobNotFound" — these won't match isENOENT(), causing missing-blob reads to surface as 500s instead of the expected 400/404.

Proposed fix

Normalize not-found errors inside each adapter so consumers never need to know the backend:

• Option A: Adapters catch provider-specific not-found and re-throw a shared StorageNotFoundError.
• Option B: Add a readFileOrNull() method that returns null instead of throwing.

Either way, isENOENT() callers (readJson, logo route, etc.) should be updated to use the normalized path.

Affected files

• lib/cast/server/storage-adapter.ts (interface + LocalFsAdapter)
• lib/cast/server/brand-loader.ts (readJson())
• app/api/brands/[slug]/logos/[id]/route.ts (error catch)

Related

• Issue perf(storage): add listPrefixes to StorageAdapter for efficient brand enumeration (from PR #51 Copilot review) #52 (listPrefixes adapter method)
• PR refactor(brands): migrate brand-loader to StorageAdapter #51 Round 3 thread docs: fix two anchor/label nits flagged in PR #5 review #7