refactor(backend): migrate db.DB to db.Conn PoolProxy across all handlers

Replace direct *pgxpool.Pool usage with PoolProxy wrapper across the entire backend:

- db.DB renamed to db.Conn (*pgxpool.Pool -> *PoolProxy)
- JWT functions now accept context.Context instead of using context.Background()
- Handler DB calls route through PoolProxy for per-test transaction support
- Fixture/helper/testdb functions accept Querier interface for decoupling
- Query ordering fixed in bookings handlers: COUNT after data query to avoid pgx conn busy
- Time truncation fixed: time.Date instead of Truncate(24*time.Hour) for week start calc
- testmain_test.go files updated with SeedBaseline and NewPoolProxy

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/auth/jwt.go

@@ -35,11 +35,11 @@ func generateJTI() (string, error) {
 }
 
 // RevokeJTI adds a JTI to the revoked set in PostgreSQL
-func RevokeJTI(jti string, expiresAt time.Time) {
-	if db.DB == nil {
+func RevokeJTI(ctx context.Context, jti string, expiresAt time.Time) {
+	if db.Conn == nil {
 		return
 	}
-	_, err := db.DB.Exec(context.Background(),
+	_, err := db.Conn.Exec(ctx,
 		`INSERT INTO revoked_jtis (jti, expires_at) VALUES ($1, $2)
 		 ON CONFLICT (jti) DO NOTHING`,
 		jti, expiresAt)
@@ -53,12 +53,12 @@ func RevokeJTI(jti string, expiresAt time.Time) {
 // Returns false if the DB is not initialized (unit tests, startup) — treating
 // the token as valid is the safer default for availability over security during
 // startup, and revoked checks are quickly re-evaluated on each request.
-func IsJTIRevoked(jti string) bool {
-	if db.DB == nil {
+func IsJTIRevoked(ctx context.Context, jti string) bool {
+	if db.Conn == nil {
 		return false
 	}
 	var exists bool
-	err := db.DB.QueryRow(context.Background(),
+	err := db.Conn.QueryRow(ctx,
 		`SELECT EXISTS(SELECT 1 FROM revoked_jtis WHERE jti = $1 AND expires_at > NOW())`,
 		jti).Scan(&exists)
 	if err != nil {
@@ -68,11 +68,11 @@ func IsJTIRevoked(jti string) bool {
 }
 
 // CleanupRevokedJTIs removes expired entries from PostgreSQL
-func CleanupRevokedJTIs() {
-	if db.DB == nil {
+func CleanupRevokedJTIs(ctx context.Context) {
+	if db.Conn == nil {
 		return
 	}
-	_, err := db.DB.Exec(context.Background(),
+	_, err := db.Conn.Exec(ctx,
 		`DELETE FROM revoked_jtis WHERE expires_at < NOW()`)
 	if err != nil {
 		fmt.Printf("WARN: Failed to cleanup revoked JTIs: %v\n", err)
@@ -85,7 +85,7 @@ func StartJTICleanup() {
 		ticker := time.NewTicker(30 * time.Minute)
 		defer ticker.Stop()
 		for range ticker.C {
-			CleanupRevokedJTIs()
+			CleanupRevokedJTIs(context.Background())
 		}
 	}()
 }
@@ -145,7 +145,7 @@ func VerifyToken(tokenString string, ctx context.Context) (userID string, role s
 		return "", "", "", fmt.Errorf("invalid jti claim")
 	}
 
-	if IsJTIRevoked(jti) {
+	if IsJTIRevoked(ctx, jti) {
 		return "", "", "", fmt.Errorf("token revoked")
 	}
 
@@ -163,7 +163,7 @@ func generateRefreshTokenString() (string, error) {
 
 // GenerateRefreshToken creates a refresh token stored in the database
 // Returns the opaque token string to return to the client
-func GenerateRefreshToken(userID string, role string) (string, error) {
+func GenerateRefreshToken(ctx context.Context, userID string, role string) (string, error) {
 	token, err := generateRefreshTokenString()
 	if err != nil {
 		return "", err
@@ -176,7 +176,7 @@ func GenerateRefreshToken(userID string, role string) (string, error) {
 		RETURNING id`
 
 	var tokenID int64
-	err = db.DB.QueryRow(context.Background(), query, userID, token, role).Scan(&tokenID)
+	err = db.Conn.QueryRow(ctx, query, userID, token, role).Scan(&tokenID)
 	if err != nil {
 		return "", fmt.Errorf("failed to store refresh token: %w", err)
 	}
@@ -194,7 +194,7 @@ func VerifyRefreshToken(ctx context.Context, tokenString string) (userID string,
 		AND NOT revoked
 		RETURNING user_id, role`
 
-	err = db.DB.QueryRow(ctx, query, tokenString).Scan(&userID, &role)
+	err = db.Conn.QueryRow(ctx, query, tokenString).Scan(&userID, &role)
 	if err != nil {
 		if errors.Is(err, pgx.ErrNoRows) {
 			return "", "", fmt.Errorf("invalid or expired refresh token")