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/db/db_dev.go

@@ -11,7 +11,7 @@ import (
 	"github.com/jackc/pgx/v5/pgxpool"
 )
 
-var DB *pgxpool.Pool
+var Conn *PoolProxy
 
 func Connect() error {
 	// Connect to Postgres inside Docker network
@@ -27,7 +27,7 @@ func Connect() error {
 		return err
 	}
 
-	DB = pool
+	Conn = NewPoolProxy(pool)
 
 	err = testDB()
 	if err != nil {
@@ -39,7 +39,7 @@ func Connect() error {
 
 func testDB() error {
 	ctx := context.Background()
-	conn, err := DB.Acquire(ctx)
+	conn, err := Conn.Acquire(ctx)
 	if err != nil {
 		return err
 	}