Make FSStore.Delete* return ErrNotFound for missing files

[damilolaedwards]

Summary

• Every Delete* method on the filesystem store passed os.Remove's raw error straight through instead of translating a missing file into the package's ErrNotFound sentinel. The S3 backend already made this distinction correctly.
• The retention watcher relies on errors.Is(err, store.ErrNotFound) to decide whether a missing file is fine to treat as already gone (and still remove the database row) versus a real failure worth retrying. On the filesystem backend that branch could never trigger, so a state whose file was already missing (manual cleanup, a crash mid-delete) could never be purged: retention logged an error and retried forever, leaving the row behind permanently.
• Fix is a single change in FSStore.removeFile, since every Delete* method routes through it.

Test plan

• go build ./... and go vet ./...
• New unit tests: each Delete* method returns a missing-file error that satisfies errors.Is(err, store.ErrNotFound); deleting an existing file still succeeds
• New integration test: a beacon state row whose file is already missing from disk is now correctly purged by the retention loop instead of being retried forever
• Confirmed all new tests fail against the pre-fix code
• Full existing pkg/store and pkg/server/persistence suites pass with no regressions