fix: improve AI feature install error handling and resource limits

Handle OOM kills (exit code 137) with actionable memory guidance,
filter ANSI/progress noise from error output, add --no-cache-dir to
pip installs, reduce download concurrency to 2, and bump default
container memory from 4g to 6g.

docker/docker-compose.yml

@@ -52,8 +52,8 @@ services:
       # - COOKIE_SECRET=
     restart: unless-stopped
     # --- Security hardening ---
-    mem_limit: 4g
-    memswap_limit: 4g
+    mem_limit: 6g
+    memswap_limit: 6g
     cpus: 4
     pids_limit: 512
     cap_drop: