fix(infra): cap observability containers' memory to protect the e2e browser (refs #123)

verify-e2e (registration.spec) timed out on the shared CI runner — locator waits
failing at different steps across attempt/retry, the signature of memory
contention, not a logic bug (this e2e has an OOM history, d5e5fa2). The three
observability containers had no memory bound; cap them (tempo/prometheus 400m,
grafana 512m — generous vs idle) so they can't starve the app stack + the
Playwright chromium. Frees back more headroom than the OTel SDK added.

refs #123

infra/docker-compose.yml

@@ -558,12 +558,16 @@ services:
       context: ./observability/tempo
     image: register-referentie/tempo:dev
     command: ["-config.file=/etc/tempo.yaml"]
+    # Cap the backplane's footprint so it can't starve the app stack + the Playwright browser on the
+    # memory-tight CI runner (verify-e2e OOM history, commit d5e5fa2). Generous vs idle (~150M).
+    mem_limit: 400m
     networks: [cg]
 
   prometheus:
     build:
       context: ./observability/prometheus
     image: register-referentie/prometheus:dev
+    mem_limit: 400m
     ports:
       - "9090:9090"
     networks: [cg]
@@ -572,6 +576,7 @@ services:
     build:
       context: ./observability/grafana
     image: register-referentie/grafana:dev
+    mem_limit: 512m
     environment:
       GF_SECURITY_ADMIN_USER: admin
       GF_SECURITY_ADMIN_PASSWORD: admin