feat(validation): relax Instagram URL validation to support all content types

- Create validateInstagramUrl utility using URL constructor
- Replace regex-based validation with hostname and protocol checks
- Support posts, reels, IGTV, and URLs with query parameters
- Add comprehensive unit tests (22 tests, all passing)
- Add integration tests for new URL formats
- Update API documentation with supported URL formats

Closes: #RelaxInstagramUrlValidation

docs/API.md

@@ -51,11 +51,36 @@ Enqueue an Instagram URL for async processing.
 }
 ```
 
+**Supported URL Formats:**
+- Posts: `https://instagram.com/p/{post-id}`
+- Posts (www): `https://www.instagram.com/p/{post-id}`
+- Reels: `https://instagram.com/reel/{reel-id}`
+- IGTV: `https://instagram.com/tv/{video-id}`
+- With query parameters: `https://instagram.com/reel/{reel-id}?utm_source=share`
+
+**URL Requirements:**
+- Must use HTTPS protocol
+- Hostname must be `instagram.com` or `www.instagram.com`
+- Any Instagram path is accepted (posts, reels, IGTV, etc.)
+- Query parameters and hash fragments are allowed
+
+**Examples:**
+```json
+// Post URL
+{ "url": "https://instagram.com/p/ABC123" }
+
+// Reel URL with tracking
+{ "url": "https://www.instagram.com/reel/DSevV5CDcNm/?utm_source=ig_web_copy_link" }
+
+// IGTV URL
+{ "url": "https://instagram.com/tv/XYZ789" }
+```
+
 **Response (201 Created):**
 ```json
 {
   "id": "550e8400-e29b-41d4-a716-446655440000",
-  "url": "https://instagram.com/p/abc123",
+  "url": "https://instagram.com/reel/DSevV5CDcNm/?utm_source=ig_web_copy_link",
   "status": "pending",
   "phases": [
     {
@@ -80,7 +105,9 @@ Enqueue an Instagram URL for async processing.
 ```
 
 **Errors:**
-- `400` - Invalid Instagram URL format
+- `400` - Invalid URL format (not a valid URL)
+- `400` - URL must use HTTPS protocol
+- `400` - URL must be from instagram.com domain
 - `400` - Missing or invalid URL parameter
 
 ### GET /api/queue