Add measured PostgreSQL query profiling

[oosuhada]

Summary

• add two query-shape-specific PostgreSQL composite indexes
• add isolated EXPLAIN (ANALYZE, BUFFERS) benchmark with safety guard
• commit measured before/after plan evidence and limitations
• add strict PostgreSQL profiling CI workflow

Measured evidence

• recent measurements p50: 10.230 ms -> 0.033 ms; buffer hits 12078 -> 14
• related cases p50: 0.162 ms -> 0.018 ms; incremental sort removed

Verification

• Ruff passed; profile tests 4/4 passed
• strict smaller CI-style profile passed
• canonical verification passed in an isolated Compose project: Python regression, held-out eval, frontend tests/build/audit/E2E, PostgreSQL/Redpanda/Neo4j integration and final gate