WIP: Add Caddy reverse proxy for dev environment

Add Caddy to docker-compose as the single entry point on port 8000:
- localhost -> /api/* stripped and proxied to CP:8080, /* to frontend:5173
- *.localhost -> proxied to CP:8080 (sandbox proxy catch-all)
- Direct /v1/*, /auth/*, /docs routes proxied to CP

Move CP from :8000 to :8080 (its default). Caddy takes :8000.
Update .env.example, vite proxy target (kept as fallback), and Makefile
dev targets (pg_isready via docker exec, frontend binds 0.0.0.0).

This is an intermediate state — needs further work for the full code
interpreter feature.

.env.example

@@ -5,13 +5,13 @@ DATABASE_URL=postgres://wrenn:wrenn@localhost:5432/wrenn?sslmode=disable
 REDIS_URL=redis://localhost:6379/0
 
 # Control Plane
-CP_LISTEN_ADDR=:8000
+CP_LISTEN_ADDR=:8080
 
 # Host Agent
 AGENT_LISTEN_ADDR=:50051
 AGENT_FILES_ROOTDIR=/var/lib/wrenn
 AGENT_HOST_INTERFACE=eth0
-AGENT_CP_URL=http://localhost:8000
+AGENT_CP_URL=http://localhost:8080
 
 # Lago (billing — external service)
 LAGO_API_URL=http://localhost:3000