Config Loading Refactor: Replace hand-rolled loader with Gestalt

[coopernetes]

Current State

The config loading system is hand-rolled:

• JettyConfigurationLoader — 3-tier YAML loader (base → local override → env vars)
• JettyConfigurationBuilder — casts raw Map<String, Object> into domain objects

Pain Points

• Partial env var coverage — only simple scalar keys; complex structures (whitelist filters, regex patterns) can't be overridden via env vars
• No profiles — no git-proxy-prod.yml / git-proxy-dev.yml activation
• No type safety — raw Map<String, Object> throughout; type mismatches surface at runtime
• No validation — bad config silently produces defaults or runtime exceptions
• No cloud config integration — no Vault, AWS SSM, or Kubernetes Secrets support

Proposed Solution: Replace with Gestalt

Gestalt is a lightweight, container-free config library that covers all current gaps natively.

Feature	Today	Gestalt
YAML loading	✅	✅
File merging / layering	✅ manual	✅ native
Env var overrides	⚠️ partial	✅ full
Profile activation	❌	✅
Type-safe POJO binding	❌	✅
Startup validation (JSR-380)	❌	✅
Cloud config (Vault, SSM, K8s)	❌	✅

Tasks

• Add gestalt-core + gestalt-yaml dependencies to jgit-proxy-server/build.gradle
• Define typed config records/POJOs (ServerConfig, ProviderConfig, CommitConfig, DatabaseConfig) to replace Map<String, Object> casts in JettyConfigurationBuilder
• Replace JettyConfigurationLoader with a GestaltBuilder composition (base → profile → local override → env vars)
• Simplify JettyConfigurationBuilder — replace all raw map casts with gestalt.getConfig(...)
• Update entry points GitProxyJettyApplication and GitProxyWithDashboardApplication to use the Gestalt-backed loader
• Add profile activation via GITPROXY_PROFILE env var