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/env.py

@@ -19,7 +19,9 @@ import ypms.models  # noqa: F401 — every model module; see src/ypms/models.py
 
 config = context.config
 if config.config_file_name is not None:
-    fileConfig(config.config_file_name)
+    # Don't disable loggers configured before Alembic runs (app, or pytest when
+    # the drift test invokes migrations in-process).
+    fileConfig(config.config_file_name, disable_existing_loggers=False)
 
 config.set_main_option("sqlalchemy.url", get_settings().database_url)
 target_metadata = Base.metadata