Simplify API: remove topics, rename session→offer

- Remove topic-based grouping and discovery
- Rename sessions to offers for clarity
- Simplify to just POST /offer, POST /answer, POST /poll
- Add version to health endpoint
- Update database schema (sessions→offers table)
- Reduce offer timeout to 1 minute
- Server version: 0.0.1

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

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

migrations/0002_remove_topics.sql

@@ -0,0 +1,22 @@
+-- Remove topics and rename sessions to offers
+-- This is a breaking change requiring a fresh database
+
+-- Drop old sessions table
+DROP TABLE IF EXISTS sessions;
+
+-- Create offers table (without topic)
+CREATE TABLE offers (
+  code TEXT PRIMARY KEY,
+  origin 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_offers_expires_at ON offers(expires_at);
+CREATE INDEX idx_offers_origin ON offers(origin);