The server was setting Access-Control-Allow-Origin: * on every route,
including the unauthenticated GET /subscription/email which returns the
subscriber's email address. Any website could query known pubkeys and
harvest emails.
Changes:
- src/env.ts: require CORS_ORIGIN env var (fail closed, no wildcard)
- src/server.ts: scope CORS middleware to browser-facing routes only,
skip /notify (server-to-server), add Vary: Origin header, import
CORS_ORIGIN from env instead of defaulting to '*'
- .env.template: document new CORS_ORIGIN variable
- test/cors.test.sh: verify CORS on browser routes, no CORS on
server-to-server routes, Vary: Origin presence
