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/testutils/helpers.go

@@ -11,27 +11,20 @@ import (
 	"net/http/httptest"
 	"testing"
 
-	"crussell/db"
-	"crussell/mw"
 	"crussell/testutils/jwt"
 	"crussell/testutils/testdb"
 
 	"github.com/jackc/pgx/v5/pgxpool"
 )
 
-// SetupTestDB resets test data by truncating tables
-// Assumes db.DB is already set by TestMain
-func SetupTestDB(t *testing.T) func() {
-	t.Helper()
+// contextKey matches mw.UserIDKey to avoid import cycle with auth package.
+type contextKey string
 
-	testdb.TruncateTables(t, db.DB)
+const userIDKey contextKey = "user_id"
 
-	return func() {}
-}
-
-// MakeRequest makes an HTTP request to a handler with optional JWT token
-// token can be user token or admin token. Pass empty string for no auth.
-func MakeRequest(handler http.Handler, method, path string, body interface{}, token string) *httptest.ResponseRecorder {
+// MakeRequest makes an HTTP request to a handler with optional JWT token.
+// ctx should carry a per-test transaction (from SetupTestTx) for PoolProxy routing.
+func MakeRequest(handler http.Handler, method, path string, body interface{}, token string, ctx context.Context) *httptest.ResponseRecorder {
 	var req *http.Request
 	if body != nil {
 		bodyBytes, _ := json.Marshal(body)
@@ -41,6 +34,8 @@ func MakeRequest(handler http.Handler, method, path string, body interface{}, to
 		req = httptest.NewRequest(method, path, nil)
 	}
 
+	req = req.WithContext(ctx)
+
 	if token != "" {
 		req.Header.Set("Authorization", "Bearer "+token)
 	}
@@ -68,25 +63,26 @@ func MakeRequestWithContext(handler http.Handler, method, path string, body inte
 	return w
 }
 
-// MakeUserRequest makes a request as an authenticated user
-// Generates a valid user token and includes it in the Authorization header
-func MakeUserRequest(handler http.Handler, method, path string, body interface{}, userID string) *httptest.ResponseRecorder {
+// MakeUserRequest makes a request as an authenticated user.
+// ctx should carry a per-test transaction (from SetupTestTx) for PoolProxy routing.
+func MakeUserRequest(handler http.Handler, method, path string, body interface{}, userID string, ctx context.Context) *httptest.ResponseRecorder {
 	token := jwt.GenerateUserToken(userID)
-	return MakeRequest(handler, method, path, body, token)
+	return MakeRequest(handler, method, path, body, token, ctx)
 }
 
-// MakeAdminRequest makes a request as an authenticated admin
-// Generates a valid admin token and includes it in the Authorization header
-func MakeAdminRequest(handler http.Handler, method, path string, body interface{}, adminID string) *httptest.ResponseRecorder {
+// MakeAdminRequest makes a request as an authenticated admin.
+// ctx should carry a per-test transaction (from SetupTestTx) for PoolProxy routing.
+func MakeAdminRequest(handler http.Handler, method, path string, body interface{}, adminID string, ctx context.Context) *httptest.ResponseRecorder {
 	token := jwt.GenerateTestToken(adminID, "admin")
-	return MakeRequest(handler, method, path, body, token)
+	return MakeRequest(handler, method, path, body, token, ctx)
 }
 
-// MakeContextRequest makes a request with user context set
-// Useful for testing handlers that check context before validating token
-func MakeContextRequest(handler http.Handler, method, path string, body interface{}, userID string) *httptest.ResponseRecorder {
-	ctx := context.WithValue(context.Background(), mw.UserIDKey, userID)
-	return MakeRequestWithContext(handler, method, path, body, ctx)
+// MakeContextRequest makes a request with user context set.
+// ctx should carry a per-test transaction (from SetupTestTx) for PoolProxy routing.
+// The user ID is layered on top of the transaction context.
+func MakeContextRequest(handler http.Handler, method, path string, body interface{}, userID string, ctx context.Context) *httptest.ResponseRecorder {
+	reqCtx := context.WithValue(ctx, userIDKey, userID)
+	return MakeRequestWithContext(handler, method, path, body, reqCtx)
 }
 
 // ParseResponseBody unmarshals the response body into dest
@@ -156,9 +152,10 @@ func GetBodyAsJSON(w *httptest.ResponseRecorder) (map[string]interface{}, error)
 	return result, err
 }
 
-// MakeRequestNoAuth makes an HTTP request without authentication (for testing unauthenticated endpoints)
-func MakeRequestNoAuth(handler http.Handler, method, path string, body interface{}) *httptest.ResponseRecorder {
-	return MakeRequest(handler, method, path, body, "")
+// MakeRequestNoAuth makes an HTTP request without authentication.
+// ctx should carry a per-test transaction (from SetupTestTx) for PoolProxy routing.
+func MakeRequestNoAuth(handler http.Handler, method, path string, body interface{}, ctx context.Context) *httptest.ResponseRecorder {
+	return MakeRequest(handler, method, path, body, "", ctx)
 }
 
 // AssertErrorStatusCode checks status code and validates error is in response body