refactor(config): unify all config types in gkg-server-config crate

Summary

Moves all configuration struct definitions into gkg-server-config so any crate in the workspace can access config types without pulling in heavy runtime dependencies (indexer, clickhouse driver, tonic, etc.).

• 20+ config types consolidated from 5 crates (clickhouse-client, indexer, health-check, gitlab-client, gkg-server) into gkg-server-config
• gkg-server-config gains only 2 new deps: config (YAML loading) and thiserror
• Runtime methods stay in their original crates: build_client() via ClickHouseConfigurationExt trait, load_tls_config() as a free function in gkg-server
• Source crates re-export types for backward compatibility
• Removed dead from_env() methods that were never called (legacy parallel config path)
• Fixed HealthCheckConfig default/serde divergence bug (serde default was missing "nats" in services list)
• Restored 15 NatsConfiguration unit tests lost during file replacement

Config loading architecture

All configuration flows through a single path: AppConfig::load(), which layers three sources in order:

1. config/default.yaml — baked into the repo, provides development defaults
2. SecretFileSource (/etc/secrets/) — reads K8s-mounted secret volumes where each key is a file (e.g. /etc/secrets/gitlab/jwt/signing_key), mapped to dot-separated config keys. Keeps credentials out of env vars and kubectl describe pod output.
3. GKG_* environment variables — Helm chart overrides for per-environment infrastructure settings (e.g. GKG_GRAPH__URL, GKG_NATS__URL). __ is the nesting separator.

Later sources override earlier ones. The removed from_env() methods were a dead parallel path that used different env var prefixes (NATS_URL vs GKG_NATS__URL) and were never called.

What moved

Types	From	To
AppConfig, SecretFileSource, GrpcConfig, MetricsConfig, TlsConfig, GitlabConfig, JwtConfig, ConfigError	gkg-server	gkg-server-config::app, grpc, metrics, tls, gitlab
ClickHouseConfiguration, ProfilingConfig, ConfigurationError	clickhouse-client	gkg-server-config::clickhouse
NatsConfiguration	indexer	gkg-server-config::nats
EngineConfiguration, HandlerConfiguration, ScheduleConfiguration, ScheduleConfig, + 10 handler/dispatcher configs	indexer	gkg-server-config::engine
HealthCheckConfig	health-check	gkg-server-config::health_check
GitlabClientConfiguration	gitlab-client	gkg-server-config::gitlab

What stays in original crates

Crate	Keeps	Why
clickhouse-client	ClickHouseConfigurationExt::build_client()	Needs ArrowClickHouseClient (orphan rule)
gkg-server	load_tls_config() free function	Needs tonic::transport
indexer	IndexerConfig struct	Aggregates config for indexer::run() — follow-up to eliminate

Follow-up

IndexerConfig still mirrors AppConfig fields and is manually constructed in main.rs. Now that AppConfig lives in the shared crate, the indexer can accept &AppConfig directly — tracked as a separate MR.