Fix hardcoded service names in database URLs

[zakkg3]

Problem

The chart currently uses hardcoded service names in the default database
URLs:

• plausible-analytics-postgresql:5432
• plausible-analytics-clickhouse:8123

This causes connection failures in multi-release deployments where the
actual service names include the release name (e.g.,
my-release-plausible-analytics-postgresql).

Solution

This PR introduces dynamic service name generation using Helm template
functions:

New helper templates:

• plausible-analytics.postgresql.serviceName - Generates correct
  PostgreSQL service name
• plausible-analytics.clickhouse.serviceName - Generates correct
  ClickHouse service name
• plausible-analytics.postgresql.defaultURL - Auto-generates PostgreSQL
  connection URL
• plausible-analytics.clickhouse.defaultURL - Auto-generates ClickHouse
  connection URL

Configuration changes:

• Database URLs are now auto-generated when left empty in values.yaml
• Added support for external database services via externalServiceName
  configuration
• Maintains full backward compatibility with explicitly configured URLs

Example

Before (hardcoded):
postgres://postgres:postgres@plausible-analytics-postgresql:5432/plausible
_db

After (dynamic):
postgres://postgres:postgres@my-release-plausible-analytics-postgresql:543
2/plausible_db

Testing

• Verified chart renders without errors via helm template
• Confirmed generated URLs use correct release-specific service names
• Validated backward compatibility with existing configurations

Breaking Changes

None - this change is fully backward compatible.

---

Files changed:

• templates/_helpers.tpl - Added helper functions for service name
  generation
• templates/secret.yaml - Updated to use helper functions when URLs not
  provided
• values.yaml - Removed hardcoded URLs and added external service support

Fixes connection issues in multi-release environments and improves chart
flexibility for various deployment scenarios.

[zakkg3]

Fixes #23