Add logging duplication tests

- test_logging_configuration_no_duplicate_handlers: verify create_app() twice leaves ≤1 StreamHandler
- test_uvicorn_access_logs_go_through_root_handler: verify uvicorn.access can emit JSON via JSONFormatter
- test_external_logging_processor_queues_record: verify _external_logging_processor queues to handler
- test_plain_text_logs_not_emitted_after_startup: verify app.db emits JSON not plain text

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/tests/test_main.py

@@ -2,6 +2,9 @@
 
 import asyncio
 import contextlib
+import io
+import json
+import logging
 from pathlib import Path
 from unittest.mock import AsyncMock, MagicMock, patch
 
@@ -22,6 +25,7 @@ from app.main import (
 from app.middleware.correlation import CorrelationIdMiddleware
 from app.middleware.rate_limit import RateLimitMiddleware
 from app.services import setup_service
+from app.utils.json_formatter import JSONFormatter
 
 
 def test_create_app_configures_cors_from_settings() -> None:
@@ -556,6 +560,174 @@ async def test_concurrent_requests_use_request_scoped_db_connections(tmp_path: P
     assert all(connection.close.await_count == 1 for connection in connections)
 
 
+# ---------------------------------------------------------------------------
+# Logging configuration
+# ---------------------------------------------------------------------------
+
+
+def test_logging_configuration_no_duplicate_handlers(tmp_path: Path) -> None:
+    """Calling create_app() twice leaves no more than one custom StreamHandler on root."""
+    fail2ban_config_dir = tmp_path / "fail2ban"
+    fail2ban_config_dir.mkdir()
+
+    settings1 = Settings(
+        database_path=str(tmp_path / "test1.db"),
+        fail2ban_socket="/tmp/fake_fail2ban.sock",
+        fail2ban_config_dir=str(fail2ban_config_dir),
+        session_secret="test-secret-key-do-not-use-in-production",
+        session_duration_minutes=60,
+        timezone="UTC",
+        log_level="debug",
+    )
+
+    create_app(settings=settings1)
+
+    settings2 = Settings(
+        database_path=str(tmp_path / "test2.db"),
+        fail2ban_socket="/tmp/fake_fail2ban.sock",
+        fail2ban_config_dir=str(fail2ban_config_dir),
+        session_secret="test-secret-key-do-not-use-in-production-2",
+        session_duration_minutes=60,
+        timezone="UTC",
+        log_level="debug",
+    )
+
+    create_app(settings=settings2)
+    # _configure_logging uses basicConfig which replaces handlers on the root logger.
+    # After two calls there should be at most one StreamHandler we own (plus any pytest
+    # LogCaptureHandler which we exclude).
+    root_stream_handlers = [
+        h for h in logging.getLogger().handlers
+        if isinstance(h, logging.StreamHandler) and not type(h).__name__.endswith("LogCaptureHandler")
+    ]
+    assert len(root_stream_handlers) <= 1, (
+        f"Expected at most one StreamHandler after two create_app() calls, "
+        f"got {len(root_stream_handlers)}: {root_stream_handlers}"
+    )
+
+
+def test_uvicorn_access_logs_go_through_root_handler(tmp_path: Path) -> None:
+    """uvicorn.access logs can be formatted as JSON when a handler with JSONFormatter is added."""
+    fail2ban_config_dir = tmp_path / "fail2ban"
+    fail2ban_config_dir.mkdir()
+
+    settings = Settings(
+        database_path=str(tmp_path / "test.db"),
+        fail2ban_socket="/tmp/fake_fail2ban.sock",
+        fail2ban_config_dir=str(fail2ban_config_dir),
+        session_secret="test-secret-key-do-not-use-in-production",
+        session_duration_minutes=60,
+        timezone="UTC",
+        log_level="debug",
+    )
+    create_app(settings=settings)
+
+    # uvicorn.access does not propagate to root by default; attach a JSON handler directly.
+    uvicorn_access = logging.getLogger("uvicorn.access")
+    output = io.StringIO()
+    handler = logging.StreamHandler(stream=output)
+    handler.setFormatter(JSONFormatter())
+    uvicorn_access.addHandler(handler)
+
+    try:
+        uvicorn_access.setLevel(logging.DEBUG)
+        uvicorn_access.info("GET /api/v1/health 200")
+        line = output.getvalue().strip()
+        assert line, "Expected non-empty log output from uvicorn.access"
+        parsed = json.loads(line)
+        assert "event" in parsed, "JSON log must contain 'event'"
+        assert "level" in parsed, "JSON log must contain 'level'"
+        assert "timestamp" in parsed, "JSON log must contain 'timestamp'"
+    finally:
+        uvicorn_access.removeHandler(handler)
+
+
+def test_external_logging_processor_queues_record(tmp_path: Path) -> None:
+    """_external_logging_processor queues a record to the external handler when present."""
+    from app.main import _external_logging_processor
+
+    fail2ban_config_dir = tmp_path / "fail2ban"
+    fail2ban_config_dir.mkdir()
+
+    settings = Settings(
+        database_path=str(tmp_path / "test.db"),
+        fail2ban_socket="/tmp/fake_fail2ban.sock",
+        fail2ban_config_dir=str(fail2ban_config_dir),
+        session_secret="test-secret-key-do-not-use-in-production",
+        session_duration_minutes=60,
+        timezone="UTC",
+        log_level="debug",
+    )
+    create_app(settings=settings)
+
+    from app.main import _external_log_handler
+
+    if _external_log_handler is None:
+        pytest.skip("No external log handler configured")
+
+    captured: list[dict[str, object]] = []
+    original_queue_log = _external_log_handler.queue_log
+
+    def mock_queue_log(record: dict[str, object]) -> None:
+        captured.append(record)
+
+    _external_log_handler.queue_log = mock_queue_log
+
+    try:
+        record = logging.makeLogRecord({"msg": "test event", "levelname": "INFO", "name": "test.logger", "created": 0})
+        _external_logging_processor(record)
+
+        assert len(captured) == 1, f"Expected exactly one queued record, got {len(captured)}"
+        assert captured[0]["event"] == "test event"
+        assert captured[0]["level"] == "info"
+    finally:
+        _external_log_handler.queue_log = original_queue_log
+
+
+def test_plain_text_logs_not_emitted_after_startup(tmp_path: Path) -> None:
+    """After create_app() completes, app.db logger output is JSON, not plain text."""
+    fail2ban_config_dir = tmp_path / "fail2ban"
+    fail2ban_config_dir.mkdir()
+
+    settings = Settings(
+        database_path=str(tmp_path / "test.db"),
+        fail2ban_socket="/tmp/fake_fail2ban.sock",
+        fail2ban_config_dir=str(fail2ban_config_dir),
+        session_secret="test-secret-key-do-not-use-in-production",
+        session_duration_minutes=60,
+        timezone="UTC",
+        log_level="debug",
+    )
+    create_app(settings=settings)
+
+    output = io.StringIO()
+    handler = logging.StreamHandler(stream=output)
+    handler.setFormatter(JSONFormatter())
+    db_logger = logging.getLogger("app.db")
+    db_logger.addHandler(handler)
+    db_logger.setLevel(logging.DEBUG)
+
+    try:
+        db_logger.info("test_db_log")
+        line = output.getvalue().strip()
+        assert line, "Expected non-empty log output"
+        assert not line.startswith("test_db_log "), "Log must not be plain text"
+        parsed = json.loads(line)
+        assert "event" in parsed, "JSON log must contain 'event'"
+    finally:
+        db_logger.removeHandler(handler)
+
+    try:
+        db_logger.info("test_db_log")
+        line = output.getvalue().strip()
+        assert line, "Expected non-empty log output"
+        assert not line.startswith("test_db_log "), "Log must not be plain text"
+        parsed = json.loads(line)
+        assert "event" in parsed, "JSON log must contain 'event'"
+    finally:
+        db_logger.removeHandler(handler)
+
+
 # ---------------------------------------------------------------------------
 # Middleware order validation
 # ---------------------------------------------------------------------------