Phase 0 milestone 0.9 (code): close plan §6/§7 control gaps

Before flipping control-mapping rows, wire the Phase 0 promises that had
no code behind them yet:

- SI-11 / C.5.7.43: app-wide exception handler returns
  {"error":"internal_error","request_id":...} and the X-Request-ID header.
  Logs exception type + stack frames, not the message — driver errors
  echo bound parameters and key values. Engine set to hide_parameters.
- AU-11 / SI-12 (plan §7 risk): create_app refuses to start if any
  mapped table lacks a retention policy.
- CM-3: migrations/IMMUTABLE.sha256 pins every merged revision; a test
  fails on any edit or on an unpinned new revision.
- migrations/env.py: fileConfig(disable_existing_loggers=False) so
  in-process Alembic (the drift test) doesn't silence app loggers.

163 tests passing; ruff, mypy, bandit, gitleaks clean.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

migrations/IMMUTABLE.sha256

@@ -0,0 +1,8 @@
+# Merged Alembic revisions and their SHA-256. NIST CM-3; CONTRIBUTING rule 1.
+# tests/test_migration_immutability.py fails if a listed file changes or a
+# revision file is missing from this list. Add a line when you add a
+# migration; never change an existing line.
+995b5def7a41c0f8bb319087675dab8dfefb2bd170d89986d07d1fe8c523d538  0001_baseline.py
+70ce3ffc67466544397843391a445790d26049d632e5fda2061a1e393a9c6612  0002_framework_tables.py
+5092d31c78a6c119b992441bd25dafc9b47bf60cf987d6cf20511ff204608d27  0003_registrants.py
+0f9551c9d94921b1b1f1e86beb9594abd2220ad1bbb1215f157cf933c2a70ee6  0004_stub_entity_tables.py