v0.1.12 — Performance + migration hotfix

Performance (measured against real 76-table store):

• db_get_schema 444 → 0 SQLite calls per invocation (batched column knowledge lookup, was N+1)
• load_config() now cached at module level — was re-parsing TOML on every tool call
• get_relationships BFS now loads all edges once, traverses in pure Python — no lock held during traversal
• save_cached_schema + discover_relationships_bulk converted to executemany

Critical hotfix on top of v0.1.11:
The v0.1.11 column-case migration crashed with PRIMARY KEY constraint violation when two mixed-case rows would lowercase to the same key (e.g. JobStatus and JOBSTATUS both → jobstatus). Affected anyone upgrading from v0.1.10 → v0.1.11 with pre-existing annotations.

Fix: dedupe in Python (last write wins) and rewrite the table atomically. Idempotent guard so the migration only runs when needed.

Tests: 185/185 pass.