feat(02-02): convert handlers to Server struct methods, remove globals

- Add Server struct with store Store and webhookSecret fields
- Add NewServer constructor
- Convert all 6 handler functions to methods on *Server
- Replace all inline SQL with s.store.X() calls
- Remove package-level globals db, mu, webhookSecret
- Remove InitDB, SetWebhookSecret, UpdateEvent, GetUpdates functions
- Update export_test.go: replace old helpers with NewTestServer, NewTestServerWithSecret, TestUpsertEvent, TestGetUpdatesMap
- Update main.go: sql.Open -> RunMigrations -> NewSQLiteStore -> NewServer -> routes

pkg/diunwebhook/export_test.go

@@ -1,19 +1,46 @@
 package diunwebhook
 
-func GetUpdatesMap() map[string]UpdateEntry {
-	m, _ := GetUpdates()
+import "database/sql"
+
+// NewTestServer constructs a Server with a fresh in-memory SQLite database.
+// Each call returns an isolated server -- tests do not share state.
+func NewTestServer() (*Server, error) {
+	db, err := sql.Open("sqlite", ":memory:")
+	if err != nil {
+		return nil, err
+	}
+	if err := RunMigrations(db); err != nil {
+		return nil, err
+	}
+	store := NewSQLiteStore(db)
+	return NewServer(store, ""), nil
+}
+
+// NewTestServerWithSecret constructs a Server with webhook authentication enabled.
+func NewTestServerWithSecret(secret string) (*Server, error) {
+	db, err := sql.Open("sqlite", ":memory:")
+	if err != nil {
+		return nil, err
+	}
+	if err := RunMigrations(db); err != nil {
+		return nil, err
+	}
+	store := NewSQLiteStore(db)
+	return NewServer(store, secret), nil
+}
+
+// TestUpsertEvent calls UpsertEvent on the server's store (for test setup).
+func (s *Server) TestUpsertEvent(event DiunEvent) error {
+	return s.store.UpsertEvent(event)
+}
+
+// TestGetUpdates calls GetUpdates on the server's store (for test assertions).
+func (s *Server) TestGetUpdates() (map[string]UpdateEntry, error) {
+	return s.store.GetUpdates()
+}
+
+// TestGetUpdatesMap is a convenience wrapper that returns the map without error.
+func (s *Server) TestGetUpdatesMap() map[string]UpdateEntry {
+	m, _ := s.store.GetUpdates()
 	return m
 }
-
-func UpdatesReset() {
-	InitDB(":memory:")
-}
-
-func ResetTags() {
-	db.Exec(`DELETE FROM tag_assignments`)
-	db.Exec(`DELETE FROM tags`)
-}
-
-func ResetWebhookSecret() {
-	SetWebhookSecret("")
-}