feat(api): admin settings API endpoints

Add four admin-only endpoints under /api/admin/settings:

- GET  /                — full config (secrets redacted)
- GET  /{module}        — single module config
- PUT  /{module}        — toggle modules + persist config overrides
- POST /{module}/test   — test external connectivity (database, storage)

PUT publishes a settings.changed SSE event. Config overrides are
persisted for future hot-reload support; changes to database/storage/
server/schemas namespaces return restart_required: true.

Wires SettingsRepository into Server struct.

Closes #99

internal/api/handlers.go

@@ -51,6 +51,7 @@ type Server struct {
 	jobDefsDir    string
 	modules       *modules.Registry
 	cfg           *config.Config
+	settings      *db.SettingsRepository
 }
 
 // NewServer creates a new API server.
@@ -77,6 +78,7 @@ func NewServer(
 	itemFiles := db.NewItemFileRepository(database)
 	dag := db.NewDAGRepository(database)
 	jobs := db.NewJobRepository(database)
+	settings := db.NewSettingsRepository(database)
 	seqStore := &dbSequenceStore{db: database, schemas: schemas}
 	partgen := partnum.NewGenerator(schemas, seqStore)
 
@@ -103,6 +105,7 @@ func NewServer(
 		jobDefsDir:    jobDefsDir,
 		modules:       registry,
 		cfg:           cfg,
+		settings:      settings,
 	}
 }