fix(streaming): report a truncated final article as corruption instead of a bare EOF

[javi11]

Summary

A user on 026bd93 reported high CPU while streaming. Their log was one client retrying the same 4760-byte tail range ~15×/s for the whole window, every attempt failing with unexpected EOF: reader ended at offset N before the requested end.

Root cause: the file's final article holds fewer bytes than its metadata claims (truncated last part), so the advertised FileSize can never be served. Since #940 the no-progress guard returned a bare io.ErrUnexpectedEOF. WebDAV had already committed a 206 with the full Content-Length, the body came up short, nothing reached the health pipeline, and the player (MKV cues live at the tail) retried forever. Each retry rebuilt the reader and fetched the final article twice.

Fix:

• truncatedTailError(at) wraps the verdict in a usenet.DataCorruptionError (NoRetry, FileOffset at the truncation point).
• Both stalled sites (Read, shared ReadAtContext) route it through classifyReadError → health update → repair trigger → CorruptedFileError (503). Follow-up requests return 404 without touching Usenet.
• The ephemeral ReadAt path applies the same verdict when a fresh reader inside the advertised size yields nothing.
• The error still unwraps to io.ErrUnexpectedEOF, so the fix(streaming): read the padded final AES block; stop rotating readers that make no progress #940 tests keep passing.

Test plan

• New tests in internal/nzbfilesystem/truncated_tail_test.go (RED before, GREEN after): Read path, ReadAt path, and health/repair trigger with the real SQLite health repo.
• go test ./internal/nzbfilesystem/... ./internal/webdav/... ./internal/usenet/... pass; go vet clean.
• Live replay against a fixture built from a real 8.9 GB file with a 4760-byte truncated last segment: first request logs "Streaming failure detected" and returns 503; a replayed storm of 420 requests all return 404 with no article fetches.
• Local golangci-lint is broken with Go 1.27 (export-data version mismatch); relying on CI lint.