fix: replace crypto/md5 with crypto/sha256 for IP hashing

backend/handlers/bookings/admin_reserve.go

@@ -2,7 +2,7 @@ package bookings
 
 import (
 	"context"
-	"crypto/md5"
+	"crypto/sha256"
 	"crussell/clock"
 	"crussell/db"
 	"crussell/handlers/scheduling"
@@ -152,7 +152,7 @@ func AdminReserveSlotHandler(w http.ResponseWriter, r *http.Request) {
 			ip = r.RemoteAddr
 		}
 	}
-	ipHash := fmt.Sprintf("%x", md5.Sum([]byte(ip)))[:8]
+	ipHash := fmt.Sprintf("%x", sha256.Sum256([]byte(ip)))[:8]
 	if _, delErr := db.Conn.Exec(r.Context(), `
 		DELETE FROM time_blockers
 		WHERE (description LIKE 'RESERVATION:admin:%' AND created_by = $1)

backend/handlers/bookings/overlap_test.go

@@ -5,7 +5,7 @@ package bookings
 import (
 	"bytes"
 	"context"
-	"crypto/md5"
+	"crypto/sha256"
 	"encoding/json"
 	"fmt"
 	"net/http"
@@ -1938,7 +1938,7 @@ func TestReserveSlot_CleansUpAnonReservation(t *testing.T) {
 
 	// Set a known IP that the test request will use
 	testIP := "192.0.2.1"
-	ipHash := fmt.Sprintf("%x", md5.Sum([]byte(testIP)))[:8]
+	ipHash := fmt.Sprintf("%x", sha256.Sum256([]byte(testIP)))[:8]
 
 	// Create an anonymous reservation (created_by = NULL) matching this IP hash
 	var blockerID string
@@ -2139,7 +2139,7 @@ func TestAdminReserveSlot_CleansUpAnonReservation(t *testing.T) {
 
 	future := weekdayTime(time.Monday, 10)
 	testIP := "192.0.2.2"
-	ipHash := fmt.Sprintf("%x", md5.Sum([]byte(testIP)))[:8]
+	ipHash := fmt.Sprintf("%x", sha256.Sum256([]byte(testIP)))[:8]
 
 	// Create an anonymous reservation matching this IP
 	var blockerID string

backend/handlers/bookings/reserve.go

@@ -1,7 +1,7 @@
 package bookings
 
 import (
-	"crypto/md5"
+	"crypto/sha256"
 	"encoding/json"
 	"fmt"
 	"log"
@@ -140,7 +140,7 @@ func ReserveSlotHandler(w http.ResponseWriter, r *http.Request) {
 	if hasAuth {
 		// Also compute IP hash to clean up anonymous reservations
 		// that may have been created before the user logged in.
-		ipHash := fmt.Sprintf("%x", md5.Sum([]byte(ip)))[:8]
+		ipHash := fmt.Sprintf("%x", sha256.Sum256([]byte(ip)))[:8]
 		if _, delErr := db.Conn.Exec(r.Context(), `
 			DELETE FROM time_blockers
 			WHERE (created_by = $1 AND description LIKE 'RESERVATION:user:%')
@@ -245,7 +245,7 @@ func ReserveSlotHandler(w http.ResponseWriter, r *http.Request) {
 		}
 	} else {
 		// Calculate ipHash from IP address
-		ipHash := fmt.Sprintf("%x", md5.Sum([]byte(ip)))[:8]
+		ipHash := fmt.Sprintf("%x", sha256.Sum256([]byte(ip)))[:8]
 
 		// ANONYMOUS: Use transaction for atomic rate cap + overlap check + insert
 		tx, err := db.Conn.Begin(r.Context())