Add polished answer notifications (#1)

Sends a branded Resend email when a question receives its first answer, records accepted and failed sends, and adds collapsed answer editing with cancel behavior.

Co-authored-by: codegirl-007 <s.raide@gmail.com>

internal/store/migrate.go

@@ -21,6 +21,20 @@ func migrateUserProfileColumns(ctx context.Context, exec execContext) error {
 	return nil
 }
 
+// migrateUserEmail adds email and a partial unique index on lower(email).
+func migrateUserEmail(ctx context.Context, exec execContext) error {
+	if _, err := exec.ExecContext(ctx, `ALTER TABLE users ADD COLUMN IF NOT EXISTS email TEXT NOT NULL DEFAULT ''`); err != nil {
+		return fmt.Errorf("add column email: %w", err)
+	}
+	if _, err := exec.ExecContext(ctx, `
+CREATE UNIQUE INDEX IF NOT EXISTS users_email_lower_uidx
+  ON users (lower(email))
+  WHERE email <> ''`); err != nil {
+		return fmt.Errorf("users_email_lower_uidx: %w", err)
+	}
+	return nil
+}
+
 type execContext interface {
 	ExecContext(ctx context.Context, query string, args ...any) (sql.Result, error)
 	QueryContext(ctx context.Context, query string, args ...any) (*sql.Rows, error)
@@ -66,6 +80,7 @@ CREATE TABLE IF NOT EXISTS schema_migrations (
 			return applySchema(ctx, exec, schemaSQL)
 		}},
 		{"002_user_profile_columns", migrateUserProfileColumns},
+		{"003_user_email", migrateUserEmail},
 	}
 	for _, m := range migrations {
 		if applied[m.version] {