feat(02-01): add Store interface and SQLiteStore implementation

- Store interface with 9 methods covering all persistence operations
- SQLiteStore implements all 9 methods with exact SQL from current handlers
- NewSQLiteStore sets MaxOpenConns(1) and PRAGMA foreign_keys = ON
- UpsertEvent uses ON CONFLICT DO UPDATE with acknowledged_at reset to NULL
- AssignTag uses INSERT OR REPLACE for tag_assignments table
- golang-migrate v4.19.1 dependency added to go.mod

pkg/diunwebhook/store.go

@@ -0,0 +1,15 @@
+package diunwebhook
+
+// Store defines all persistence operations. Implementations must be safe
+// for concurrent use from HTTP handlers.
+type Store interface {
+	UpsertEvent(event DiunEvent) error
+	GetUpdates() (map[string]UpdateEntry, error)
+	AcknowledgeUpdate(image string) (found bool, err error)
+	ListTags() ([]Tag, error)
+	CreateTag(name string) (Tag, error)
+	DeleteTag(id int) (found bool, err error)
+	AssignTag(image string, tagID int) error
+	UnassignTag(image string) error
+	TagExists(id int) (bool, error)
+}