fix(logging): T-5 — production logs INFO to stdout

docker logs tatlock has been empty for months and the /app/logs
mount untouched since 2025-12. Not a broken handler: the stdout
StreamHandler was correct all along, but the production default
level was WARNING ("minimal noise") and a healthy service warns
roughly never — startup, backend flavor detection and the wrapper's
balancing/compaction signals were all suppressed, which is exactly
what made the serving cutover unverifiable from logs on 2026-09-12.

Production now defaults to INFO; LOG_LEVEL stays the override for
when noise is ever the real problem; development keeps DEBUG. The
mount was always decorative — no file handler exists, docker logs is
the log. Three tests pin the level table, the WARNING-revert
mutation shown to fail its test.

Also filed T-7: the full suite flaked once and passed on rerun for
the second time — next occurrence keeps the failing run's output.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/core/test_logging_config.py

@@ -241,6 +241,41 @@ class TestLoggingConfiguration:
         with patch.object(config, "ENVIRONMENT", Environment.PRODUCTION):
             assert config.log_format == "json"
 
+    def test_production_default_level_is_info(self):
+        """Production logs INFO to stdout (T-5).
+
+        The old WARNING default made `docker logs` empty for months — a
+        healthy service warns roughly never, so startup, flavor
+        detection and the wrapper signals were all suppressed and the
+        serving cutover could not be verified from logs.
+        """
+        from src.core.config import Environment, config
+
+        with (
+            patch.object(config, "ENVIRONMENT", Environment.PRODUCTION),
+            patch.object(config, "LOG_LEVEL", None),
+        ):
+            assert config.effective_log_level == "INFO"
+
+    def test_log_level_env_still_quiets_production(self):
+        """LOG_LEVEL stays the override for when noise is the problem."""
+        from src.core.config import Environment, config
+
+        with (
+            patch.object(config, "ENVIRONMENT", Environment.PRODUCTION),
+            patch.object(config, "LOG_LEVEL", "WARNING"),
+        ):
+            assert config.effective_log_level == "WARNING"
+
+    def test_development_default_stays_debug(self):
+        from src.core.config import Environment, config
+
+        with (
+            patch.object(config, "ENVIRONMENT", Environment.DEVELOPMENT),
+            patch.object(config, "LOG_LEVEL", None),
+        ):
+            assert config.effective_log_level == "DEBUG"
+
     def test_multiple_loggers_independent(self):
         """Test multiple loggers are independent."""
         logger1 = get_logger("test.logger1")