feat: Implement content-based offer IDs with SHA-256 hashing

- Added hash-id.ts utility for SHA-256 content hashing
- Offer IDs now generated from hash of {sdp, topics} (sorted)
- Removed peerId from hash (inferred from authentication)
- Server generates deterministic IDs for idempotent offer creation
- Updated SQLite and D1 storage implementations
- Removed optional id field from CreateOfferRequest
- Same offer content always produces same ID

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

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

src/storage/hash-id.ts

@@ -0,0 +1,37 @@
+/**
+ * Generates a content-based offer ID using SHA-256 hash
+ * Creates deterministic IDs based on offer content (sdp, topics)
+ * PeerID is not included as it's inferred from authentication
+ * Uses Web Crypto API for compatibility with both Node.js and Cloudflare Workers
+ *
+ * @param sdp - The WebRTC SDP offer
+ * @param topics - Array of topic strings
+ * @returns SHA-256 hash of the sanitized offer content
+ */
+export async function generateOfferHash(
+  sdp: string,
+  topics: string[]
+): Promise<string> {
+  // Sanitize and normalize the offer content
+  // Only include core offer content (not peerId - that's inferred from auth)
+  const sanitizedOffer = {
+    sdp,
+    topics: [...topics].sort(), // Sort topics for consistency
+  };
+
+  // Create non-prettified JSON string
+  const jsonString = JSON.stringify(sanitizedOffer);
+
+  // Convert string to Uint8Array for hashing
+  const encoder = new TextEncoder();
+  const data = encoder.encode(jsonString);
+
+  // Generate SHA-256 hash
+  const hashBuffer = await crypto.subtle.digest('SHA-256', data);
+
+  // Convert hash to hex string
+  const hashArray = Array.from(new Uint8Array(hashBuffer));
+  const hashHex = hashArray.map(b => b.toString(16).padStart(2, '0')).join('');
+
+  return hashHex;
+}