Server gap pass: invite-partner subcommand, CLI and drain tests, push info log (D-113, D-114)

- kvitt-server invite-partner: a partner invite by the admin as E-12 does (cancels the pending one, 72 h, link with
  ?from=<admin>), exit 2 household full, 3 no admin; README lines in server/ and infra/.
- Tests: invite-partner, migrate (schema at SCHEMA_VERSION, idempotent), healthcheck (200, 503, nothing listening),
  serve SIGTERM drain (in-flight request finishes, new connections refused, exit 0; gives up after SHUTDOWN_DRAIN).
- Push sender: one info line "push sent" per message with push_type, devices_sent, devices_targeted and 8-character
  device-id tails (SPEC §8.7 aggregates); per-device line stays at debug.
- tools/contracts-check.py: stale TODO(spec) replaced by the §9.11 CS-5 citation; shapes updated for invite_kind
  (D-086), app_version_name (D-090) and fid (D-096), 134 checks pass.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TQd3pEP9UeN2uifbHknQ7B

infra/README.md

@@ -79,6 +79,7 @@ The `bootstrap` and `pair-device` subcommands (SPEC §10.4, AC-1.1, AC-1.6) arri
 C="docker compose -f infra/dev/compose.yaml --env-file infra/dev/.env.example"   # or --env-file infra/dev/.env
 $C exec server kvitt-server bootstrap                      # first admin invite: code, link, expiry
 $C exec server kvitt-server pair-device --user <user_id>   # extra device invite for an existing user
+$C exec server kvitt-server invite-partner                 # partner invite by the admin, as E-12 (D-113)
 $C run --rm server migrate                                 # when the server is stopped (entrypoint is kvitt-server)
 ```