Update Helm chart values and template inline documentation to reflect current implementation

## Objective

Review and update all inline comments, parameter docstrings, and template descriptions across the Helm chart (`values.yaml`, `values.schema.json`, `_helpers.tpl`, and `templates/*.yaml`) to accurately reflect the current capabilities, configuration parameters, and production defaults of the server.

## Context

As the status-list-server evolved, the Helm chart received substantial enhancements:
- Support for Workload Identity across AWS, Azure, and GCP (#390, #471).
- Immutable digest-based image deployments and fallback tag logic.
- Observability and SLO threshold configurations (`slo:` blocks).
- Multi-database decoupling (Postgres and upcoming MySQL support in #498).
- Diverse secret mounting options (External Secrets Operator, file mounts, fallback Secrets).

However, many inline comments in `helm/chart/values.yaml` and template headers in `templates/*.yaml` are sparse, outdated, or focus solely on legacy AWS/Postgres setups. This makes it challenging for operators to understand available configuration options directly from the chart files.

## Deliverables

- [ ] **`values.yaml` Inline Documentation**:
- Audit and expand descriptions for every configuration section (`statuslist`, `postgres`, `mysql`, `serviceAccount`, `ingress`, `secretMounts`, `slo`, `monitoring`).
- Clearly document Workload Identity setup requirements for AWS EKS, GCP GKE, and Azure AKS.
- Document digest vs. tag image resolution rules.
- [ ] **`values.schema.json` Updates**:
- Ensure all `description` and `title` attributes in `values.schema.json` match the updated `values.yaml` docstrings.
- [ ] **Template Inline Comments**:
- Review and document template logic in `templates/deployment.yaml`, `templates/network-policy.yaml`, `templates/secret.yaml`, `templates/prometheusrule.yaml`, and `templates/_helpers.tpl`.
- Clarify helper macro behaviors (e.g. database host determination, secret key selection).
- [ ] **Validation**:
- Ensure `helm lint ./helm/chart` passes cleanly.
- Verify `local-ci.sh` Helm checks succeed.

## Acceptance Criteria

- [ ] Every configurable property in `values.yaml` has comprehensive, accurate inline documentation.
- [ ] Multi-cloud Workload Identity, multi-database, and secret mount options are clearly explained.
- [ ] `helm lint ./helm/chart` passes without errors or warnings.

## References

- Sub-EPIC: #503 (Sprint 3: Vendor-Neutral Deployment, Multi-Database Support & Performance Validation)
- Baseline EPIC: #141 (Status List Server Hardening & Production Readiness)
- Related Helm Tickets: #498, #508