feat: remaining DB indexes for structured queries

[ajianaz]

Summary

Add remaining database indexes for structured queries. Partially addressed by #666 (merged). Verify existing and add missing.

Acceptance Criteria

Functional

• Indexes for all WHERE clause columns
• Query plans show index usage

Verification

sqlite3 uteke.db "EXPLAIN QUERY PLAN SELECT * FROM memories WHERE memory_type = 'fact'"
sqlite3 uteke.db ".indices memories"

Tests

• Fresh DB migration test
• Existing DB migration: adds without data loss

Workflow

• Branch: feat/db-indexes → develop via PR
• Assignee: CTO
• Depends: Verify against merged feat(api): add POST /memory/pin and POST /memory/importance endpoints #666

[ajianaz]

Duplicate of #657 (completed). Same feat already merged.