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>

migrations/0001_add_peer_id.sql

@@ -0,0 +1,21 @@
+-- Drop old sessions table with 'info' column
+DROP TABLE IF EXISTS sessions;
+
+-- Create sessions table with peer_id column
+CREATE TABLE sessions (
+  code TEXT PRIMARY KEY,
+  origin TEXT NOT NULL,
+  topic TEXT NOT NULL,
+  peer_id TEXT NOT NULL CHECK(length(peer_id) <= 1024),
+  offer TEXT NOT NULL,
+  answer TEXT,
+  offer_candidates TEXT NOT NULL DEFAULT '[]',
+  answer_candidates TEXT NOT NULL DEFAULT '[]',
+  created_at INTEGER NOT NULL,
+  expires_at INTEGER NOT NULL
+);
+
+-- Create indexes for efficient queries
+CREATE INDEX idx_expires_at ON sessions(expires_at);
+CREATE INDEX idx_origin_topic ON sessions(origin, topic);
+CREATE INDEX idx_origin_topic_expires ON sessions(origin, topic, expires_at);