Phase 4 — Date polls, both uses

One mechanism, two uses. **With a target** → a reschedule that moves that session. **No target** → pre-signup, minting game days from winning dates with a win rule. This *is* availability polling. Hermuz's `surveys` (candidate dates → canonize → game day → carry available players over) generalises into `date_polls` with a nullable target and per-date outcomes.

Players propose, GM confirms and triggers. Max one open poll per session.

## In order
1. #34 — Schema — date_polls, poll_dates, poll_responses; minimal game_days
1. #35 — Date poll post — multi-select availability + organiser-only Canonise
1. #36 — Targeted polls — /reschedule, Suggest another day, and moving a session
1. #37 — Untargeted polls — minting game days from winning dates with a win rule
1. #38 — Auto-resolve, per-campaign opt-in