Add pruning and retention policy for devii_lessons table to prevent unbounded growth

Summary

The devii_lessons table has grown to 3827 records (soft-deleted) with no pruning, retention, or deduplication policy. Every reflect() call stores a permanent lesson used by recall() queries, causing performance degradation and accumulation of duplicate/irrelevant lessons.

Steps to Reproduce

1. Call reflect() multiple times for the same user/guest session.
2. Observe that each call inserts a new record into devii_lessons without any limit or deduplication.
3. Run recall() queries and note that old, duplicate, or irrelevant lessons are still returned.
4. Check the table row count; it grows unbounded per account/session.

Expected Behaviour

• The devii_lessons table should have a maximum lesson count per user/guest session (e.g., 500 active lessons).
• Lessons older than a configurable number of days should be automatically pruned.
• Similar or duplicate lessons should be merged or deduplicated instead of stacking.
• A mechanism (e.g., quality/thumbs-up system) should allow irrelevant lessons to be silently dropped.
• recall() queries should remain performant as the table size is bounded.

Actual Behaviour

• The table contains 3827 records (soft-deleted) with no cap, retention policy, or deduplication.
• recall() queries slow down as the table grows.
• Duplicate lessons about the same topic (e.g., container sync issues) accumulate.
• Old, irrelevant lessons continue to match in recall results.
• The database size is 44 KB and grows with every session.

Environment

Not provided.

Reported by retoor via DevPlace.