Schema: date_polls, poll_dates, poll_responses; minimal game_days

- [ ] `date_polls` — nullable `target_session_id`, `game_id`, `game_day_kind`, **`win_rule`** (`min_players` | `quorum_of_roster` | `best_available` | `organiser_picks`), `win_threshold`, status, `opened_by`, `closes_at`, channel/message ids, `auto_resolve`.
- [ ] `poll_dates` — ≤10 per poll, each with an **`outcome`** (open/won/lost/withdrawn).
- [ ] `poll_responses` — poll_date_id, user_id, available (bool), responded_at.
- [ ] Partial unique index enforcing **max one open poll per target session**.
- [ ] `game_days` arrives here in minimal form (id, kind, date, title, state) because the untargeted poll mints them; capacity, venue, host, `game_id` and seating come in phase 5.

Start by finding the existing database schema or migration entry points in the project, since the issue does not name files. Add the date_polls, poll_dates, poll_responses, and minimal game_days structures with the listed fields and constraints. Done means the schema includes the max-one-open-poll partial unique index and the poll date limit/outcome requirements are represented.

索引モデルが issue の本文から書いたものです。