feat: add versioned artifact DAG reconciliation for #1679

[Even1115]

Summary

• add a versioned, content-addressed processing artifact store with canonical keys, payload validation, checksums, fail-open recovery, singleflight, and Redis-backed leases
• cache deterministic DocReader, embedding, chat, and VLM processing stages while keeping live, streaming, query, tool, and multimodal operations out of cache
• introduce stable chunk and derived-artifact identities, attempt fencing, and desired-state reconciliation so reprocessing publishes new output before exact stale cleanup
• track Neo4j contributions per stable chunk and processing attempt for precise graph reconciliation
• add PostgreSQL/SQLite migrations plus MySQL and ParadeDB initialization schemas

Why

Knowledge reprocessing could repeat expensive provider calls, generate unstable identifiers, expose delete-first availability windows, and allow stale attempts to overwrite or remove newer output. This change models reusable intermediate results as versioned artifacts and makes publication generation-aware and non-destructive.

Impact

• unchanged processing inputs can reuse verified artifacts across attempts
• chunk and derived-output IDs remain stable across insertion and reordering
• provider failures preserve the last published output
• successful empty output is distinguished from failure and can reconcile stale state exactly
• stale attempts are fenced before provider work and publication

Validation

Passed before publishing this draft:

• artifact key, codec, identity, lease, runtime, fence, and reconciliation unit tests
• repository processing-artifact and concurrent attempt-fence tests
• service stable-ID, image prompt, table, and reconciliation tests
• SQLite migration tests
• container and Neo4j repository compile checks
• git diff --check

Still pending before marking ready for review:

• final full regression and race-test pass after the local machine restart
• live Neo4j integration verification
• live PostgreSQL/MySQL migration verification

Related to #1679.