migration: Convert peer_id to username in offers and ice_candidates

This migration aligns the D1 database schema with the unified Ed25519
authentication system that replaced the dual peerId/secret system.

Changes:
- Renames peer_id to username in offers table
- Renames answerer_peer_id to answerer_username in offers table
- Renames peer_id to username in ice_candidates table
- Adds service_fqn column to offers table
- Updates all indexes and foreign keys

migrations/0008_peer_id_to_username.sql

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+-- Migration: Convert peer_id to username in offers and ice_candidates tables
+-- This migration aligns the database with the unified Ed25519 authentication system
+
+-- Step 1: Recreate offers table with username instead of peer_id
+CREATE TABLE offers_new (
+  id TEXT PRIMARY KEY,
+  username TEXT NOT NULL,
+  service_id TEXT,
+  service_fqn TEXT,
+  sdp TEXT NOT NULL,
+  created_at INTEGER NOT NULL,
+  expires_at INTEGER NOT NULL,
+  last_seen INTEGER NOT NULL,
+  answerer_username TEXT,
+  answer_sdp TEXT,
+  answered_at INTEGER,
+  FOREIGN KEY (username) REFERENCES usernames(username) ON DELETE CASCADE,
+  FOREIGN KEY (answerer_username) REFERENCES usernames(username) ON DELETE SET NULL
+);
+
+-- Step 2: Migrate data (if any) - peer_id becomes username
+-- Note: This assumes peer_id values were already usernames in practice
+INSERT INTO offers_new (id, username, service_id, service_fqn, sdp, created_at, expires_at, last_seen, answerer_username, answer_sdp, answered_at)
+SELECT id, peer_id as username, service_id, NULL as service_fqn, sdp, created_at, expires_at, last_seen, answerer_peer_id as answerer_username, answer_sdp, answered_at
+FROM offers;
+
+-- Step 3: Drop old offers table
+DROP TABLE offers;
+
+-- Step 4: Rename new table
+ALTER TABLE offers_new RENAME TO offers;
+
+-- Step 5: Recreate indexes
+CREATE INDEX idx_offers_username ON offers(username);
+CREATE INDEX idx_offers_service ON offers(service_id);
+CREATE INDEX idx_offers_expires ON offers(expires_at);
+CREATE INDEX idx_offers_last_seen ON offers(last_seen);
+CREATE INDEX idx_offers_answerer ON offers(answerer_username);
+
+-- Step 6: Recreate ice_candidates table with username instead of peer_id
+CREATE TABLE ice_candidates_new (
+  id INTEGER PRIMARY KEY AUTOINCREMENT,
+  offer_id TEXT NOT NULL,
+  username TEXT NOT NULL,
+  role TEXT NOT NULL CHECK(role IN ('offerer', 'answerer')),
+  candidate TEXT NOT NULL,
+  created_at INTEGER NOT NULL,
+  FOREIGN KEY (offer_id) REFERENCES offers(id) ON DELETE CASCADE,
+  FOREIGN KEY (username) REFERENCES usernames(username) ON DELETE CASCADE
+);
+
+-- Step 7: Migrate ICE candidates data
+INSERT INTO ice_candidates_new (offer_id, username, role, candidate, created_at)
+SELECT offer_id, peer_id as username, role, candidate, created_at
+FROM ice_candidates;
+
+-- Step 8: Drop old ice_candidates table
+DROP TABLE ice_candidates;
+
+-- Step 9: Rename new table
+ALTER TABLE ice_candidates_new RENAME TO ice_candidates;
+
+-- Step 10: Recreate indexes
+CREATE INDEX idx_ice_offer ON ice_candidates(offer_id);
+CREATE INDEX idx_ice_username ON ice_candidates(username);
+CREATE INDEX idx_ice_role ON ice_candidates(role);
+CREATE INDEX idx_ice_created ON ice_candidates(created_at);