feat: add core security utilities for checksum-based share links

- Add HMAC-SHA256 checksum generation and validation (shareChecksum.ts)
- Add PDF magic bytes validation to prevent file spoofing (pdfValidator.ts)
- Add IP-based rate limiting for upload abuse prevention (uploadRateLimiter.ts)
- Update BillingLocation interface with shareTTL and shareFirstVisitedAt fields
- Add environment variables for share link security and TTL configuration

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

app/lib/uploadRateLimiter.ts

@@ -0,0 +1,71 @@
+/**
+ * Simple in-memory rate limiter for upload attempts
+ * Tracks by IP address
+ */
+
+interface RateLimitEntry {
+  count: number;
+  resetAt: number; // Unix timestamp
+}
+
+// In-memory store (use Redis for production multi-instance setups)
+const rateLimitStore = new Map<string, RateLimitEntry>();
+
+/**
+ * Check if IP address is rate limited
+ * @returns { allowed: boolean, remaining: number }
+ */
+export function checkUploadRateLimit(ipAddress: string): { allowed: boolean; remaining: number; resetIn: number } {
+
+  const maxUploads = parseInt(process.env.UPLOAD_RATE_LIMIT_PER_IP || '5', 10);
+  const windowMs = parseInt(process.env.UPLOAD_RATE_LIMIT_WINDOW_MS || '3600000', 10); // 1 hour
+
+  const now = Date.now();
+  const key = `upload:${ipAddress}`;
+
+  let entry = rateLimitStore.get(key);
+
+  // Clean up expired entry or create new one
+  if (!entry || now > entry.resetAt) {
+    entry = {
+      count: 0,
+      resetAt: now + windowMs
+    };
+    rateLimitStore.set(key, entry);
+  }
+
+  // Check if limit exceeded
+  if (entry.count >= maxUploads) {
+    return {
+      allowed: false,
+      remaining: 0,
+      resetIn: Math.ceil((entry.resetAt - now) / 1000) // seconds
+    };
+  }
+
+  // Increment counter
+  entry.count++;
+  rateLimitStore.set(key, entry);
+
+  return {
+    allowed: true,
+    remaining: maxUploads - entry.count,
+    resetIn: Math.ceil((entry.resetAt - now) / 1000)
+  };
+}
+
+/**
+ * Periodic cleanup of expired entries (prevent memory leak)
+ * Call this occasionally (e.g., every hour)
+ */
+export function cleanupRateLimitStore() {
+  const now = Date.now();
+  for (const [key, entry] of rateLimitStore.entries()) {
+    if (now > entry.resetAt) {
+      rateLimitStore.delete(key);
+    }
+  }
+}
+
+// Auto-cleanup every 10 minutes
+setInterval(cleanupRateLimitStore, 10 * 60 * 1000);