docs(observability): document GlitchTip services in DEPLOYMENT.md and C4 diagram

Adds GlitchTip env vars to the observability env var table, extends the
services table, and adds a first-run section with superuser creation and
project setup steps. Updates the C4 L2 container diagram with GlitchTip
and Redis containers and their relationships.

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

docs/architecture/c4/l2-containers.puml

@@ -25,6 +25,8 @@ System_Boundary(observability, "Observability Stack (docker-compose.observabilit
     Container(promtail, "Promtail", "grafana/promtail:3.4.2", "Ships Docker container logs to Loki via Docker SD.")
     Container(tempo, "Tempo", "grafana/tempo:2.7.2", "Distributed trace storage. OTLP gRPC receiver on port 4317 (archiv-net). Grafana queries traces on port 3200 (obs-net). All ports internal only.")
     Container(grafana, "Grafana", "grafana/grafana-oss:11.6.1", "Unified observability UI — dashboards, logs, traces. Datasources (Prometheus, Loki, Tempo) and three dashboards are auto-provisioned.")
+    Container(glitchtip, "GlitchTip", "glitchtip/glitchtip:v4", "Sentry-compatible error tracker. Receives frontend + backend error events, groups by fingerprint, provides issue UI with stack traces.")
+    Container(obs_redis, "Redis", "redis:7-alpine", "Celery task queue for GlitchTip async workers.")
 }
 
 Rel(user, caddy, "HTTPS", "TLS 1.2/1.3")
@@ -43,5 +45,7 @@ Rel(backend, tempo, "Sends distributed traces via OTLP", "gRPC / OTLP / port 431
 Rel(grafana, prometheus, "Queries metrics", "HTTP 9090")
 Rel(grafana, loki, "Queries logs", "HTTP 3100")
 Rel(grafana, tempo, "Queries traces", "HTTP 3200")
+Rel(glitchtip, db, "Stores error events in glitchtip DB", "PostgreSQL / archiv-net")
+Rel(obs_glitchtip_worker, obs_redis, "Processes Celery tasks", "Redis / obs-net")
 
 @enduml