feat: implement db helpers and postgres pool/transactor

- Add is_blocked to core.users (002_core_tables.sql)
- Add is_active to activity.sessions for soft deletes (005_activity_tables.sql)
- Implement UserRepo: List, GetByID, GetByName, Create, Update, Delete
- Implement MimeRepo: List, GetByID, GetByName
- Implement SessionRepo: Create, GetByTokenHash, ListByUser,
  UpdateLastActivity, Delete, DeleteByUserID
- Session deletes are soft (SET is_active = false); is_active is a
  SQL-only filter, not mapped to the domain type

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

backend/internal/db/postgres/mime_repo.go

@@ -0,0 +1,97 @@
+package postgres
+
+import (
+	"context"
+	"errors"
+	"fmt"
+
+	"github.com/jackc/pgx/v5"
+	"github.com/jackc/pgx/v5/pgxpool"
+
+	"tanabata/backend/internal/domain"
+	"tanabata/backend/internal/port"
+)
+
+type mimeRow struct {
+	ID        int16  `db:"id"`
+	Name      string `db:"name"`
+	Extension string `db:"extension"`
+}
+
+func toMIMEType(r mimeRow) domain.MIMEType {
+	return domain.MIMEType{
+		ID:        r.ID,
+		Name:      r.Name,
+		Extension: r.Extension,
+	}
+}
+
+// MimeRepo implements port.MimeRepo using PostgreSQL.
+type MimeRepo struct {
+	pool *pgxpool.Pool
+}
+
+// NewMimeRepo creates a MimeRepo backed by pool.
+func NewMimeRepo(pool *pgxpool.Pool) *MimeRepo {
+	return &MimeRepo{pool: pool}
+}
+
+var _ port.MimeRepo = (*MimeRepo)(nil)
+
+func (r *MimeRepo) List(ctx context.Context) ([]domain.MIMEType, error) {
+	const sql = `SELECT id, name, extension FROM core.mime_types ORDER BY name`
+
+	q := connOrTx(ctx, r.pool)
+	rows, err := q.Query(ctx, sql)
+	if err != nil {
+		return nil, fmt.Errorf("MimeRepo.List: %w", err)
+	}
+	collected, err := pgx.CollectRows(rows, pgx.RowToStructByName[mimeRow])
+	if err != nil {
+		return nil, fmt.Errorf("MimeRepo.List scan: %w", err)
+	}
+
+	result := make([]domain.MIMEType, len(collected))
+	for i, row := range collected {
+		result[i] = toMIMEType(row)
+	}
+	return result, nil
+}
+
+func (r *MimeRepo) GetByID(ctx context.Context, id int16) (*domain.MIMEType, error) {
+	const sql = `SELECT id, name, extension FROM core.mime_types WHERE id = $1`
+
+	q := connOrTx(ctx, r.pool)
+	rows, err := q.Query(ctx, sql, id)
+	if err != nil {
+		return nil, fmt.Errorf("MimeRepo.GetByID: %w", err)
+	}
+	row, err := pgx.CollectOneRow(rows, pgx.RowToStructByName[mimeRow])
+	if err != nil {
+		if errors.Is(err, pgx.ErrNoRows) {
+			return nil, domain.ErrNotFound
+		}
+		return nil, fmt.Errorf("MimeRepo.GetByID scan: %w", err)
+	}
+	m := toMIMEType(row)
+	return &m, nil
+}
+
+func (r *MimeRepo) GetByName(ctx context.Context, name string) (*domain.MIMEType, error) {
+	const sql = `SELECT id, name, extension FROM core.mime_types WHERE name = $1`
+
+	q := connOrTx(ctx, r.pool)
+	rows, err := q.Query(ctx, sql, name)
+	if err != nil {
+		return nil, fmt.Errorf("MimeRepo.GetByName: %w", err)
+	}
+	row, err := pgx.CollectOneRow(rows, pgx.RowToStructByName[mimeRow])
+	if err != nil {
+		if errors.Is(err, pgx.ErrNoRows) {
+			return nil, domain.ErrUnsupportedMIME
+		}
+		return nil, fmt.Errorf("MimeRepo.GetByName scan: %w", err)
+	}
+	m := toMIMEType(row)
+	return &m, nil
+}