fix(infra): pin GlitchTip image to 6.1.6 (v4 tag never existed)

glitchtip/glitchtip:v4 is not a real tag — GlitchTip does not use a
v-prefix in its Docker image versioning. Latest stable release is 6.1.6.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-compose.observability.yml

@@ -184,7 +184,7 @@ services:
       - obs-net
 
   obs-glitchtip:
-    image: glitchtip/glitchtip:v4
+    image: glitchtip/glitchtip:6.1.6
     container_name: obs-glitchtip
     restart: unless-stopped
     depends_on:
@@ -207,7 +207,7 @@ services:
       - obs-net
 
   obs-glitchtip-worker:
-    image: glitchtip/glitchtip:v4
+    image: glitchtip/glitchtip:6.1.6
     container_name: obs-glitchtip-worker
     restart: unless-stopped
     command: ./bin/run-celery-with-beat.sh