feat: implement Havenllo single-board kanban (Go+SQLite backend, Preact/Vite frontend)

- Single fixed 'Haven' board: lists (To do/In progress/Done) + cards, no multi-board
- Go 1.24 backend, CGO-free modernc.org/sqlite, embedded SPA, REST /api
- Preact + Vite + TS frontend, native HTML5 drag-and-drop, optimistic UI, dark Haven theme
- PVC (nfs-client) for SQLite, root container on NFS, simple nginx ingress havenllo.haven
- Dockerfile multi-arch build, Gitea CI via pipeline-actions@main

internal/database/database.go

@@ -0,0 +1,37 @@
+package database
+
+import (
+	"context"
+	"database/sql"
+	"fmt"
+	"os"
+	"path/filepath"
+
+	_ "modernc.org/sqlite"
+)
+
+// Open configures the one SQLite connection used by the single-replica app and
+// migrates it before returning it to callers.
+func Open(ctx context.Context, path string) (*sql.DB, error) {
+	dir := filepath.Dir(path)
+	if dir != "." && dir != "" {
+		if err := os.MkdirAll(dir, 0o755); err != nil {
+			return nil, fmt.Errorf("create database directory: %w", err)
+		}
+	}
+	db, err := sql.Open("sqlite", path)
+	if err != nil {
+		return nil, fmt.Errorf("open database: %w", err)
+	}
+	db.SetMaxOpenConns(1)
+	db.SetMaxIdleConns(1)
+	if _, err := db.ExecContext(ctx, "PRAGMA foreign_keys = ON; PRAGMA busy_timeout = 5000; PRAGMA journal_mode = WAL;"); err != nil {
+		_ = db.Close()
+		return nil, fmt.Errorf("configure database: %w", err)
+	}
+	if err := migrate(ctx, db); err != nil {
+		_ = db.Close()
+		return nil, err
+	}
+	return db, nil
+}