feat: add component-aware RFC 5424 application logging system

- Modify Rfc5424Formatter to read decnet_component from LogRecord
  and use it as RFC 5424 APP-NAME field (falls back to 'decnet')
- Add get_logger(component) factory in decnet/logging/__init__.py
  with _ComponentFilter that injects decnet_component on each record
- Wire all five layers to their component tag:
    cli -> 'cli', engine -> 'engine', api -> 'api' (api.py, ingester,
    routers), mutator -> 'mutator', collector -> 'collector'
- Add structured INFO/DEBUG/WARNING/ERROR log calls throughout each
  layer per the defined vocabulary; DEBUG calls are suppressed unless
  DECNET_DEVELOPER=true
- Add tests/test_logging.py covering factory, filter, formatter
  component-awareness, fallback behaviour, and level gating

decnet/web/api.py

@@ -1,5 +1,4 @@
 import asyncio
-import logging
 import os
 from contextlib import asynccontextmanager
 from typing import Any, AsyncGenerator, Optional
@@ -11,12 +10,13 @@ from pydantic import ValidationError
 from fastapi.middleware.cors import CORSMiddleware
 
 from decnet.env import DECNET_CORS_ORIGINS, DECNET_DEVELOPER, DECNET_INGEST_LOG_FILE
+from decnet.logging import get_logger
 from decnet.web.dependencies import repo
 from decnet.collector import log_collector_worker
 from decnet.web.ingester import log_ingestion_worker
 from decnet.web.router import api_router
 
-log = logging.getLogger(__name__)
+log = get_logger("api")
 ingestion_task: Optional[asyncio.Task[Any]] = None
 collector_task: Optional[asyncio.Task[Any]] = None
 
@@ -25,9 +25,11 @@ collector_task: Optional[asyncio.Task[Any]] = None
 async def lifespan(app: FastAPI) -> AsyncGenerator[None, None]:
     global ingestion_task, collector_task
 
+    log.info("API startup initialising database")
     for attempt in range(1, 6):
         try:
             await repo.initialize()
+            log.debug("API startup DB initialised attempt=%d", attempt)
             break
         except Exception as exc:
             log.warning("DB init attempt %d/5 failed: %s", attempt, exc)
@@ -40,11 +42,13 @@ async def lifespan(app: FastAPI) -> AsyncGenerator[None, None]:
         # Start background ingestion task
         if ingestion_task is None or ingestion_task.done():
             ingestion_task = asyncio.create_task(log_ingestion_worker(repo))
+            log.debug("API startup ingest worker started")
 
         # Start Docker log collector (writes to log file; ingester reads from it)
         _log_file = os.environ.get("DECNET_INGEST_LOG_FILE", DECNET_INGEST_LOG_FILE)
         if _log_file and (collector_task is None or collector_task.done()):
             collector_task = asyncio.create_task(log_collector_worker(_log_file))
+            log.debug("API startup collector worker started log_file=%s", _log_file)
         elif not _log_file:
             log.warning("DECNET_INGEST_LOG_FILE not set — Docker log collection disabled.")
     else:
@@ -52,7 +56,7 @@ async def lifespan(app: FastAPI) -> AsyncGenerator[None, None]:
 
     yield
 
-    # Shutdown background tasks
+    log.info("API shutdown cancelling background tasks")
     for task in (ingestion_task, collector_task):
         if task and not task.done():
             task.cancel()
@@ -62,6 +66,7 @@ async def lifespan(app: FastAPI) -> AsyncGenerator[None, None]:
                 pass
             except Exception as exc:
                 log.warning("Task shutdown error: %s", exc)
+    log.info("API shutdown complete")
 
 
 app: FastAPI = FastAPI(