feat: initial scaffold of kanban_cpp v2 (issue 0130)

Frontend C++ ImGui (main.cpp + 4 paneles) + backend Go (HTTP + SQLite + fsnotify + SSE).
Reusa parse/scan/watch funcs del registry (issue 0130a).

backend/db.go

@@ -0,0 +1,51 @@
+package main
+
+import (
+	"database/sql"
+	"fmt"
+	"io/fs"
+	"sort"
+	"strings"
+
+	_ "github.com/mattn/go-sqlite3"
+)
+
+func openDB(path string) (*sql.DB, error) {
+	dsn := fmt.Sprintf("file:%s?_journal_mode=WAL&_foreign_keys=on", path)
+	db, err := sql.Open("sqlite3", dsn)
+	if err != nil {
+		return nil, err
+	}
+	if err := db.Ping(); err != nil {
+		return nil, err
+	}
+	return db, nil
+}
+
+func applyMigrations(db *sql.DB) error {
+	entries, err := fs.ReadDir(migrationsFS, "migrations")
+	if err != nil {
+		return err
+	}
+	names := []string{}
+	for _, e := range entries {
+		if !e.IsDir() && strings.HasSuffix(e.Name(), ".sql") {
+			names = append(names, e.Name())
+		}
+	}
+	sort.Strings(names)
+	for _, n := range names {
+		b, err := fs.ReadFile(migrationsFS, "migrations/"+n)
+		if err != nil {
+			return err
+		}
+		if _, err := db.Exec(string(b)); err != nil {
+			low := strings.ToLower(err.Error())
+			if strings.Contains(low, "duplicate column") || strings.Contains(low, "already exists") {
+				continue
+			}
+			return fmt.Errorf("%s: %w", n, err)
+		}
+	}
+	return nil
+}