fix(api): gate embedded Docker-log collector on DECNET_EMBED_COLLECTOR

The API lifespan unconditionally spawned log_collector_worker,
appending every container line to DECNET_INGEST_LOG_FILE. On hosts
that also run decnet-collector.service (installed by 'decnet init')
that's two tailers writing the same events to the same file — the
ingester then inserts each event twice and the dashboard shows every
command duplicated.

Add DECNET_EMBED_COLLECTOR (default false), matching the existing
DECNET_EMBED_PROFILER and DECNET_EMBED_SNIFFER pattern directly
above this block. Single-process dev setups without systemd can flip
it on to restore the all-in-one behaviour; multi-process production
gets the single-writer invariant by default.

decnet/env.py

@@ -70,6 +70,15 @@ DECNET_EMBED_PROFILER: bool = os.environ.get("DECNET_EMBED_PROFILER", "").lower(
 # workers sniffing the same interface — duplicated events and wasted CPU.
 DECNET_EMBED_SNIFFER: bool = os.environ.get("DECNET_EMBED_SNIFFER", "").lower() == "true"
 
+# Set to "true" to embed the Docker log collector inside the API process.
+# Leave unset (default) when `decnet-collector.service` (or a standalone
+# `decnet collect --daemon`) is running — embedding both yields two
+# tailers appending every container log line to the ingest file, which
+# the ingester then inserts into the DB twice. Single-process dev
+# setups without systemd units can flip this on to get the old all-in
+# -one behaviour.
+DECNET_EMBED_COLLECTOR: bool = os.environ.get("DECNET_EMBED_COLLECTOR", "").lower() == "true"
+
 # Set to "true" to mount the Pyinstrument ASGI middleware on the FastAPI app.
 # Produces per-request HTML flamegraphs under ./profiles/. Off by default so
 # production and normal dev runs pay zero profiling overhead.

decnet/web/api.py

@@ -14,6 +14,7 @@ from fastapi.middleware.cors import CORSMiddleware
 from decnet.env import (
     DECNET_CORS_ORIGINS,
     DECNET_DEVELOPER,
+    DECNET_EMBED_COLLECTOR,
     DECNET_EMBED_PROFILER,
     DECNET_EMBED_SNIFFER,
     DECNET_INGEST_LOG_FILE,
@@ -87,13 +88,25 @@ async def lifespan(app: FastAPI) -> AsyncGenerator[None, None]:
             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)
+        # Start Docker log collector (writes to log file; ingester reads from it).
+        # Gated on DECNET_EMBED_COLLECTOR: when `decnet-collector.service` (or
+        # any other standalone collector) is running, embedding a second tailer
+        # here writes every container line twice — the ingester then inserts
+        # the same event into the DB twice, which surfaces as duplicate rows
+        # on the dashboard.
         _log_file = os.environ.get("DECNET_INGEST_LOG_FILE", DECNET_INGEST_LOG_FILE)
+        if DECNET_EMBED_COLLECTOR:
             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)
+                log.info(
+                    "API startup: embedded collector started "
+                    "(DECNET_EMBED_COLLECTOR=true) log_file=%s",
+                    _log_file,
+                )
             elif not _log_file:
-            log.warning("DECNET_INGEST_LOG_FILE not set — Docker log collection disabled.")
+                log.warning("DECNET_INGEST_LOG_FILE not set — embedded collector disabled.")
+        else:
+            log.debug("API startup: collector not embedded — expecting standalone daemon")
 
         # Start attacker profile rebuild worker only when explicitly requested.
         # Default is OFF because `decnet deploy` always starts a standalone