Remove origin-based session bucketing

- Remove origin column from offers table
- Remove getOrigin() function and X-Rondevu-Global header support
- Simplify all storage methods to use only offer code
- Add migration 0003_remove_origin.sql
- Update error messages to remove "origin mismatch" wording

This simplifies the system by removing origin-based isolation.
All offers are now identified solely by their unique code/ID.

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

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

migrations/0003_remove_origin.sql

@@ -0,0 +1,29 @@
+-- Migration: Remove origin column from offers table
+-- This simplifies offer lookup to only use offer codes
+-- Origin-based bucketing is no longer needed
+
+-- Create new offers table without origin column
+CREATE TABLE offers_new (
+  code TEXT PRIMARY KEY,
+  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
+);
+
+-- Copy data from old table
+INSERT INTO offers_new (code, peer_id, offer, answer, offer_candidates, answer_candidates, created_at, expires_at)
+SELECT code, peer_id, offer, answer, offer_candidates, answer_candidates, created_at, expires_at
+FROM offers;
+
+-- Drop old table
+DROP TABLE offers;
+
+-- Rename new table
+ALTER TABLE offers_new RENAME TO offers;
+
+-- Recreate index
+CREATE INDEX IF NOT EXISTS idx_offers_expires_at ON offers(expires_at);