feat(03-02): add Docker Compose postgres profiles and build-tagged test helper

- compose.yml: add postgres service with profiles, healthcheck, pg_isready
- compose.yml: add DATABASE_URL env var and conditional depends_on (required: false)
- compose.yml: add postgres-data volume; default deploy remains SQLite-only
- compose.dev.yml: add postgres service with port 5432 exposed for local dev
- compose.dev.yml: add DATABASE_URL env var and conditional depends_on
- pkg/diunwebhook/postgres_test.go: build-tagged NewTestPostgresServer helper

pkg/diunwebhook/postgres_test.go

@@ -0,0 +1,29 @@
+//go:build postgres
+
+package diunwebhook
+
+import (
+	"database/sql"
+	"os"
+
+	_ "github.com/jackc/pgx/v5/stdlib"
+)
+
+// NewTestPostgresServer constructs a Server backed by a PostgreSQL database.
+// Requires a running PostgreSQL instance. Set TEST_DATABASE_URL to override
+// the default connection string.
+func NewTestPostgresServer() (*Server, error) {
+	databaseURL := os.Getenv("TEST_DATABASE_URL")
+	if databaseURL == "" {
+		databaseURL = "postgres://diun:diun@localhost:5432/diundashboard_test?sslmode=disable"
+	}
+	db, err := sql.Open("pgx", databaseURL)
+	if err != nil {
+		return nil, err
+	}
+	if err := RunPostgresMigrations(db); err != nil {
+		return nil, err
+	}
+	store := NewPostgresStore(db)
+	return NewServer(store, ""), nil
+}