feat: complete OTEL tracing across all services with pipeline bridge and docs

Extends tracing to every remaining module: all 23 API route handlers,
correlation engine, sniffer (fingerprint/p0f/syslog), prober (jarm/hassh/tcpfp),
profiler behavioral analysis, logging subsystem, engine, and mutator.

Bridges the ingester→SSE trace gap by persisting trace_id/span_id columns on
the logs table and creating OTEL span links in the SSE endpoint. Adds log-trace
correlation via _TraceContextFilter injecting otel_trace_id into Python LogRecords.

Includes development/docs/TRACING.md with full span reference (76 spans),
pipeline propagation architecture, quick start guide, and troubleshooting.

decnet/web/router/stats/api_get_stats.py

@@ -2,6 +2,7 @@ from typing import Any
 
 from fastapi import APIRouter, Depends
 
+from decnet.telemetry import traced as _traced
 from decnet.web.dependencies import require_viewer, repo
 from decnet.web.db.models import StatsResponse
 
@@ -10,5 +11,6 @@ router = APIRouter()
 
 @router.get("/stats", response_model=StatsResponse, tags=["Observability"],
     responses={401: {"description": "Could not validate credentials"}, 422: {"description": "Validation error"}},)
+@_traced("api.get_stats")
 async def get_stats(user: dict = Depends(require_viewer)) -> dict[str, Any]:
     return await repo.get_stats_summary()