Switch the subscribe endpoint from POST to an idempotent PUT so clients can
always upsert without a stateful lookup first.
- Add GET /subscription/email?pubkey= lookup so clients can avoid re-POSTing.
- insertSubscription no longer clears confirmed_at unless the email address
changes; a frequency change keeps the existing confirmation.
- registerSubscription only sends a confirmation email when the subscription
is new, unconfirmed, or the email address changed.
- Update integration test and README for the PUT endpoint.
- test/integration.sh: runs full E2E flow against fresh server
- pnpm test and pnpm test:server scripts added
- Root endpoint handles missing web UI gracefully (returns JSON)
- database.ts reads DATA_DIR env var for test isolation
- .gitignore updated for test artifacts
