refactor: clean up legacy UUID v4, use UUID v7 everywhere

Migrations:
- 016, 017, 019: Replace gen_random_uuid() with uuid_generate_v7()
- All UUID tables now use custom uuid_generate_v7() function

Backend:
- Create internal/utils/uuid/uuid.go with V7() function (RFC 9562)
- Update storage_handler.go to use internal uuid.V7()
- Remove dependency on google/uuid for file naming

All new UUIDs in the system are now UUID v7 (time-ordered)

backend/internal/handlers/storage_handler.go

@@ -8,8 +8,8 @@ import (
 	"strings"
 	"time"
 
-	"github.com/google/uuid"
 	"github.com/rede5/gohorsejobs/backend/internal/infrastructure/storage"
+	"github.com/rede5/gohorsejobs/backend/internal/utils/uuid"
 )
 
 // StorageHandler handles file storage operations
@@ -111,7 +111,7 @@ func (h *StorageHandler) GenerateUploadURL(w http.ResponseWriter, r *http.Reques
 
 	// Generate unique key
 	ext := filepath.Ext(req.Filename)
-	uniqueID := uuid.New().String()
+	uniqueID := uuid.V7()
 	timestamp := time.Now().Format("20060102")
 	key := fmt.Sprintf("%s/%s/%s%s", folder, timestamp, uniqueID, ext)
 

backend/internal/utils/uuid/uuid.go

@@ -0,0 +1,64 @@
+package uuid
+
+import (
+	"crypto/rand"
+	"encoding/binary"
+	"fmt"
+	"time"
+)
+
+// V7 generates a UUID v7 (time-ordered) following RFC 9562
+// Format: tttttttt-tttt-7xxx-yxxx-xxxxxxxxxxxx
+// Where: t = timestamp (48 bits), 7 = version, y = variant, x = random
+func V7() string {
+	// Get current timestamp in milliseconds
+	timestamp := time.Now().UnixMilli()
+
+	// Generate 10 random bytes
+	randomBytes := make([]byte, 10)
+	rand.Read(randomBytes)
+
+	// Build UUID bytes (16 total)
+	var bytes [16]byte
+
+	// First 6 bytes: timestamp (48 bits, big-endian)
+	binary.BigEndian.PutUint32(bytes[0:4], uint32(timestamp>>16))
+	binary.BigEndian.PutUint16(bytes[4:6], uint16(timestamp))
+
+	// Next 2 bytes: version (4 bits = 7) + random (12 bits)
+	bytes[6] = 0x70 | (randomBytes[0] & 0x0F) // version 7
+	bytes[7] = randomBytes[1]
+
+	// Last 8 bytes: variant (2 bits = 10) + random (62 bits)
+	bytes[8] = 0x80 | (randomBytes[2] & 0x3F) // variant RFC 4122
+	copy(bytes[9:16], randomBytes[3:10])
+
+	// Format as UUID string
+	return fmt.Sprintf("%08x-%04x-%04x-%04x-%012x",
+		binary.BigEndian.Uint32(bytes[0:4]),
+		binary.BigEndian.Uint16(bytes[4:6]),
+		binary.BigEndian.Uint16(bytes[6:8]),
+		binary.BigEndian.Uint16(bytes[8:10]),
+		bytes[10:16],
+	)
+}
+
+// IsValid checks if a string is a valid UUID (any version)
+func IsValid(s string) bool {
+	// Basic UUID format check: xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
+	if len(s) != 36 {
+		return false
+	}
+	for i, c := range s {
+		if i == 8 || i == 13 || i == 18 || i == 23 {
+			if c != '-' {
+				return false
+			}
+		} else {
+			if !((c >= '0' && c <= '9') || (c >= 'a' && c <= 'f') || (c >= 'A' && c <= 'F')) {
+				return false
+			}
+		}
+	}
+	return true
+}

backend/migrations/016_create_notifications_table.sql

@@ -1,5 +1,5 @@
 CREATE TABLE IF NOT EXISTS notifications (
-    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    id UUID PRIMARY KEY DEFAULT uuid_generate_v7(),
     user_id INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
     type VARCHAR(50) NOT NULL, -- info, success, warning, error
     title VARCHAR(255) NOT NULL,

backend/migrations/017_create_tickets_table.sql

@@ -2,7 +2,7 @@ DROP TABLE IF EXISTS ticket_messages;
 DROP TABLE IF EXISTS tickets;
 
 CREATE TABLE tickets (
-    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    id UUID PRIMARY KEY DEFAULT uuid_generate_v7(),
     user_id INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
     subject VARCHAR(255) NOT NULL,
     status VARCHAR(50) NOT NULL DEFAULT 'open', -- open, in_progress, closed
@@ -15,7 +15,7 @@ CREATE INDEX idx_tickets_user_id ON tickets(user_id);
 CREATE INDEX idx_tickets_status ON tickets(status);
 
 CREATE TABLE ticket_messages (
-    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    id UUID PRIMARY KEY DEFAULT uuid_generate_v7(),
     ticket_id UUID NOT NULL REFERENCES tickets(id) ON DELETE CASCADE,
     user_id INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE, -- Sender
     message TEXT NOT NULL,

backend/migrations/019_create_job_payments_table.sql

@@ -2,7 +2,7 @@
 -- Description: Track payments for job postings
 
 CREATE TABLE IF NOT EXISTS job_payments (
-    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    id UUID PRIMARY KEY DEFAULT uuid_generate_v7(),
     job_id INT NOT NULL,
     user_id INT,