Schema: games, campaign_members, signups (campaign forming), audit_log; full campaigns columns

Fill out the domain for many campaigns.

- [ ] `games` — name, min/max players, default duration; referenced by campaigns and (later) single game days.
- [ ] `campaigns` gains: `recurrence_anchor` + `interval_weeks`, quorum, capacity, `max_sessions`, **`first_session_number`** (session numbers need seeding because history starts empty), lifecycle state.
- [ ] `campaign_members` — the roster: campaign_id, user_id, role (gm/player), joined_at, character_name.
- [ ] `signups` — target_type (`campaign_forming` now; `game_day` in phase 5), target_id, user_id, state, position, character_name — with the **CHECK constraint that a signup never targets a session**.
- [ ] `audit_log` — who changed what from the console.

No files or tests are named. Start by reading the existing schema and migration conventions, then map the requested games, campaigns, campaign_members, signups, and audit_log tables and campaign columns. Done means the listed fields, relationships, lifecycle data, seeded session numbering, audit data, and the signup CHECK constraint are represented in the schema.

由索引模型根据 Issue 内容生成。