Persist SQL and DuckDB artifacts in storage database

[skgbafa]

Summary

• Adds a database_artifact migration/model and a SeaORM repository backed by storage.database for SQL/DuckDB artifact payloads and metadata.
• Treats local SQL/DuckDB files as cache: cold opens hydrate from the durable artifact row, and stale local-only cache files are removed when no durable row exists.
• Persists checkpointed/exported SQL and DuckDB artifacts after successful writes/imports before returning success; durable-save failure returns an error and evicts local cache state.
• Adds service tests for SQL restart with empty cache, DuckDB restart/export with empty cache, DuckDB import restart with empty cache, and durable persistence failure.

Verification

• cargo test -p tinycloud-core - passed, 53 tests + doc tests.
• cargo test -p tinycloud-node - passed, 31 lib tests, 3 main tests + doc tests.
• git diff --check - passed.

Residual Risks / Follow-ups

• Artifacts are stored as database blobs for this vertical slice. Very large SQL/DuckDB databases may need a follow-up backend that stores payload bytes through the configured block store while keeping metadata/revisions in storage.database.
• This does not automatically migrate pre-existing local-only SQL/DuckDB cache files into the durable artifact table; an explicit one-time migration/import flow may be needed for legacy local data.
• Invocation metadata is still committed before the SQL/DuckDB service path runs. Artifact persistence failure now fails the request, but the invocation metadata row can already exist.