Replace KV storage with D1, add peer_id field, simplify README

Storage changes:
- Remove KV storage adapter
- Add D1 storage adapter for Cloudflare Workers
- Update schema to use peer_id instead of info field
- Add database migrations for D1

Documentation:
- Simplify README to be more concise
- Update deployment instructions for D1
- Add D1_SETUP.md explaining migration from KV
- Update DEPLOYMENT.md with D1 setup steps

API changes:
- Replace info field with peerId in session creation
- Update all storage interfaces and implementations

🤖 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);