Implement Phase 0+1: project setup, config, database, migrations

- Go module, Makefile, .golangci.yaml, .gitignore, example config
- TOML config loading with validation
- SQLite database with WAL, foreign keys, busy timeout
- Schema migrations: users, webauthn_credentials, notebooks, pages,
  strokes, share_links with indexes and cascading deletes
- 4 tests: open+migrate, idempotent, foreign keys, cascade delete

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/db/db.go

@@ -0,0 +1,29 @@
+package db
+
+import (
+	"database/sql"
+	"fmt"
+
+	_ "modernc.org/sqlite"
+)
+
+func Open(path string) (*sql.DB, error) {
+	db, err := sql.Open("sqlite", path)
+	if err != nil {
+		return nil, fmt.Errorf("open database: %w", err)
+	}
+
+	pragmas := []string{
+		"PRAGMA journal_mode = WAL",
+		"PRAGMA foreign_keys = ON",
+		"PRAGMA busy_timeout = 5000",
+	}
+	for _, p := range pragmas {
+		if _, err := db.Exec(p); err != nil {
+			_ = db.Close()
+			return nil, fmt.Errorf("exec %q: %w", p, err)
+		}
+	}
+
+	return db, nil
+}