Add structured backend logging with pino

Wires leveled, structured logging (pretty in dev, JSON in prod) through
the server lifecycle, HTTP/WS request handling, run engine, auth, db,
and config loading. CLI command output is left untouched since it's
user-facing terminal UX, not backend logs.

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

src/app/api/runs/[runId]/cancel/route.ts

@@ -5,6 +5,9 @@ import { requireAuth, unauthorizedResponse } from "@/lib/auth/guard";
 import { getDb } from "@/lib/db/client";
 import { runs } from "@/lib/db/schema";
 import { cancelRun } from "@/lib/runner/registry";
+import { logger } from "@/lib/logger";
+
+const log = logger.child({ mod: "api" });
 
 export async function POST(
   request: Request,
@@ -27,11 +30,16 @@ export async function POST(
 
   const cancelled = cancelRun(runId);
   if (!cancelled) {
+    log.warn(
+      { runId },
+      "cancel requested for a run not tracked by this process",
+    );
     return Response.json(
       { error: "Run is not active in this server process" },
       { status: 409 },
     );
   }
 
+  log.info({ runId, requestedBy: auth.identity }, "cancel requested via API");
   return Response.json({ status: "cancelling" }, { status: 202 });
 }