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

src/lib/server/validation/instagram-url.ts

@@ -0,0 +1,79 @@
+/**
+ * Instagram URL Validation Utility
+ * 
+ * Validates that a URL is from Instagram's domain and uses HTTPS.
+ * Accepts all Instagram URL formats (posts, reels, IGTV, etc.).
+ */
+
+export interface ValidationResult {
+	valid: boolean;
+	error?: string;
+}
+
+/**
+ * Validate Instagram URL
+ * 
+ * Accepts:
+ * - https://instagram.com/p/{post-id}
+ * - https://www.instagram.com/p/{post-id}
+ * - https://instagram.com/reel/{reel-id}
+ * - https://instagram.com/tv/{tv-id}
+ * - Any Instagram URL with query parameters
+ * 
+ * Rejects:
+ * - Non-HTTPS URLs (http://)
+ * - Non-Instagram domains
+ * - Invalid URL format
+ * - Subdomains other than www
+ * 
+ * @param url - The URL to validate
+ * @returns Validation result with valid flag and optional error message
+ * 
+ * @example
+ * ```typescript
+ * const result = validateInstagramUrl('https://instagram.com/reel/ABC123?utm_source=share');
+ * if (!result.valid) {
+ *   console.error(result.error);
+ * }
+ * ```
+ */
+export function validateInstagramUrl(url: string): ValidationResult {
+	// Validate URL is a string
+	if (typeof url !== 'string' || url.trim() === '') {
+		return {
+			valid: false,
+			error: 'URL must be a non-empty string'
+		};
+	}
+
+	// Parse URL
+	let urlObj: URL;
+	try {
+		urlObj = new URL(url);
+	} catch (e) {
+		return {
+			valid: false,
+			error: 'Invalid URL format'
+		};
+	}
+
+	// Validate protocol (must be HTTPS)
+	if (urlObj.protocol !== 'https:') {
+		return {
+			valid: false,
+			error: 'Instagram URL must use HTTPS protocol'
+		};
+	}
+
+	// Validate hostname (must be instagram.com or www.instagram.com)
+	const validHostnames = ['instagram.com', 'www.instagram.com'];
+	if (!validHostnames.includes(urlObj.hostname)) {
+		return {
+			valid: false,
+			error: 'URL must be from instagram.com domain'
+		};
+	}
+
+	// Valid Instagram URL
+	return { valid: true };
+}