Add lint, service config, and test helper templates for HAF apps

New templates added to haf_app_testing.gitlab-ci.yml:

Lint templates:
- .haf_app_lint_bash - Bash script linting with shellcheck
- .haf_app_lint_sql - SQL linting with sqlfluff (PostgreSQL)

Service container configuration:
- .haf_service_config - HAF instance configuration variables
- .postgrest_service_config - PostgREST service configuration

Test helpers (composable with !reference):
- .haf_app_wait_for_postgres - Wait for PostgreSQL readiness
- .haf_app_wait_for_postgrest - Wait for PostgREST API
- .haf_app_wait_for_sync - Wait for HAF to reach target block
- .haf_app_extract_test_cache - Extract cache for test jobs

These templates reduce duplication across HAF apps (balance_tracker,
reputation_tracker, haf_block_explorer) by providing reusable building
blocks for common CI patterns.

Also adds CI-IMPROVEMENT-PLAN.md documenting the analysis and future
improvement roadmap for HAF app CI configurations.