feat(backend): implement db helpers and postgres pool/transactor

db/db.go: TxFromContext/ContextWithTx for transaction propagation,
Querier interface (QueryRow/Query/Exec), ScanRow generic helper,
ClampLimit/ClampOffset pagination guards.

db/postgres/postgres.go: NewPool with ping validation, Transactor
backed by pgxpool (BeginTx → fn → commit/rollback), connOrTx helper
that returns the active transaction from context or falls back to pool.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/internal/db/db.go

@@ -0,0 +1,70 @@
+// Package db provides shared helpers used by all database adapters.
+package db
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/jackc/pgx/v5"
+	"github.com/jackc/pgx/v5/pgconn"
+)
+
+// txKey is the context key used to store an active transaction.
+type txKey struct{}
+
+// TxFromContext returns the pgx.Tx stored in ctx by the Transactor, along
+// with a boolean indicating whether a transaction is active.
+func TxFromContext(ctx context.Context) (pgx.Tx, bool) {
+	tx, ok := ctx.Value(txKey{}).(pgx.Tx)
+	return tx, ok
+}
+
+// ContextWithTx returns a copy of ctx that carries tx.
+// Called by the Transactor before invoking the user function.
+func ContextWithTx(ctx context.Context, tx pgx.Tx) context.Context {
+	return context.WithValue(ctx, txKey{}, tx)
+}
+
+// Querier is the common query interface satisfied by both *pgxpool.Pool and
+// pgx.Tx, allowing repo helpers to work with either.
+type Querier interface {
+	QueryRow(ctx context.Context, sql string, args ...any) pgx.Row
+	Query(ctx context.Context, sql string, args ...any) (pgx.Rows, error)
+	Exec(ctx context.Context, sql string, args ...any) (pgconn.CommandTag, error)
+}
+
+// ScanRow executes a single-row query against q and scans the result using
+// scan. It wraps pgx.ErrNoRows so callers can detect missing rows without
+// importing pgx directly.
+func ScanRow[T any](ctx context.Context, q Querier, sql string, args []any, scan func(pgx.Row) (T, error)) (T, error) {
+	row := q.QueryRow(ctx, sql, args...)
+	val, err := scan(row)
+	if err != nil {
+		var zero T
+		if err == pgx.ErrNoRows {
+			return zero, fmt.Errorf("%w", pgx.ErrNoRows)
+		}
+		return zero, fmt.Errorf("ScanRow: %w", err)
+	}
+	return val, nil
+}
+
+// ClampLimit enforces the [1, max] range on limit, returning def when limit
+// is zero or negative.
+func ClampLimit(limit, def, max int) int {
+	if limit <= 0 {
+		return def
+	}
+	if limit > max {
+		return max
+	}
+	return limit
+}
+
+// ClampOffset returns 0 for negative offsets.
+func ClampOffset(offset int) int {
+	if offset < 0 {
+		return 0
+	}
+	return offset
+}

backend/internal/db/postgres/postgres.go

@@ -0,0 +1,67 @@
+// Package postgres provides the PostgreSQL implementations of the port interfaces.
+package postgres
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/jackc/pgx/v5"
+	"github.com/jackc/pgx/v5/pgxpool"
+
+	"tanabata/backend/internal/db"
+)
+
+// NewPool creates and validates a *pgxpool.Pool from the given connection URL.
+// The pool is ready to use; the caller is responsible for closing it.
+func NewPool(ctx context.Context, url string) (*pgxpool.Pool, error) {
+	pool, err := pgxpool.New(ctx, url)
+	if err != nil {
+		return nil, fmt.Errorf("pgxpool.New: %w", err)
+	}
+	if err := pool.Ping(ctx); err != nil {
+		pool.Close()
+		return nil, fmt.Errorf("postgres ping: %w", err)
+	}
+	return pool, nil
+}
+
+// Transactor implements port.Transactor using a pgxpool.Pool.
+type Transactor struct {
+	pool *pgxpool.Pool
+}
+
+// NewTransactor creates a Transactor backed by pool.
+func NewTransactor(pool *pgxpool.Pool) *Transactor {
+	return &Transactor{pool: pool}
+}
+
+// WithTx begins a transaction, stores it in ctx, and calls fn. If fn returns
+// an error the transaction is rolled back; otherwise it is committed.
+func (t *Transactor) WithTx(ctx context.Context, fn func(ctx context.Context) error) error {
+	tx, err := t.pool.BeginTx(ctx, pgx.TxOptions{})
+	if err != nil {
+		return fmt.Errorf("begin tx: %w", err)
+	}
+
+	txCtx := db.ContextWithTx(ctx, tx)
+
+	if err := fn(txCtx); err != nil {
+		_ = tx.Rollback(ctx)
+		return err
+	}
+
+	if err := tx.Commit(ctx); err != nil {
+		return fmt.Errorf("commit tx: %w", err)
+	}
+	return nil
+}
+
+// connOrTx returns the pgx.Tx stored in ctx by WithTx, or the pool itself when
+// no transaction is active. The returned value satisfies db.Querier and can be
+// used directly for queries and commands.
+func connOrTx(ctx context.Context, pool *pgxpool.Pool) db.Querier {
+	if tx, ok := db.TxFromContext(ctx); ok {
+		return tx
+	}
+	return pool
+}