feat(probes): add startup probe for handling slow initial startup

Add startupProbe to give Jellyfin sufficient time to start, especially
with large media libraries or slow storage. This prevents Kubernetes
from killing the pod during initial startup phase.

Changes:
- Add startupProbe configuration in values.yaml with sensible defaults
  - failureThreshold: 30 (allows up to 5 minutes for startup)
  - periodSeconds: 10
  - Uses tcpSocket check on http port
- Update deployment.yaml to include startupProbe from values
- Bump chart version 2.5.0 -> 2.5.1
- Add changelog annotation for Artifact Hub

Benefits:
- Prevents pod restarts during slow initial startup (large media libraries)
- Liveness and readiness probes remain responsive after successful startup
- Fully configurable and can be disabled by setting startupProbe: null
- No breaking changes, backward compatible

The startup probe gives Jellyfin up to 5 minutes (30 attempts * 10 seconds)
to complete initialization. After the first success, liveness and readiness
probes take over with their normal timing.

Co-Authored-By: Claude <noreply@anthropic.com>
Signed-off-by: Aleksei Sviridkin <f@lex.la>

charts/jellyfin/templates/deployment.yaml

@@ -108,6 +108,10 @@ spec:
               hostPort: 1900
               protocol: UDP
             {{- end }}
+          {{- with .Values.startupProbe }}
+          startupProbe:
+            {{- toYaml . | nindent 12 }}
+          {{- end }}
           livenessProbe:
             {{- toYaml .Values.livenessProbe | nindent 12 }}
           readinessProbe: