refactor: Phase 5 - split API package into focused files
Reorganized internal/api/ package to follow single-concern principle:
- api/factory.go (new file, 257 lines)
- Extracted component initialization from server.go
- initializeComponents(), setupLogger(), initExperimentManager()
- initTaskQueue(), initDatabase(), initDatabaseSchema()
- initSecurity(), initJupyterServiceManager(), initAuditLogger()
- api/middleware.go (new file, 31 lines)
- Extracted wrapWithMiddleware() - security middleware chain
- Centralized auth, rate limiting, CORS, security headers
- api/server.go (reduced from 446 to 212 lines)
- Now focused on Server lifecycle: NewServer, Start, WaitForShutdown, Close
- Removed initialization logic (moved to factory.go)
- Removed middleware wrapper (moved to middleware.go)
- api/metrics_middleware.go (existing, 64 lines)
- Already had wrapWithMetrics(), left in place
Lines redistributed: ~180 lines from monolithic server.go
Build status: Compiles successfully
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