Draw the data model in the wiki

[elgorro]

Closes #36

The domain model landed in #33 with 47 entities and no map — reading the EF configurations was the only way to see how communities, sessions and matches fit together.

This adds a Data model section to the wiki: an overview page plus one ER diagram per area, drawn from src/Ssabba.Infrastructure/Configurations.

• _index.md — flowchart of how the areas hang off Community/CommunityMember, and the schema-wide conventions (Guid v7 keys, int-stored enums, AmountMinor + char(3) money, the four soft-deleted entities).
• Identity and communities — shared-PK profile, memberships, invites and join requests, seasons. Notes that rating lives on the membership, not the player.
• Venues and sessions — the GiST no-overlap exclusion on reservations, recurring sessions, the plus-one self-reference and the waitlist check constraint, weather, polls.
• Matches and ratings — why a match snapshots its scoring rules, MatchAppearance as the rating journal, and the RatingAppliedAt worker queue.
• Operations and money — double-entry ledger, its check constraints and the reversal self-FK, dues, funding, equipment loans.
• Privacy and notifications — append-only consent, the deliberately FK-less AuditEvent.EntityId, the outbox and its partial pending index.

No configuration or theme change: hugo-book already renders plain ```mermaid fences.

Verification

• hugo --minify builds clean; every relref resolves.
• All nine diagrams parse against mermaid 11, so none render blank.
• Cardinality and delete behaviour spot-checked against SessionParticipantConfiguration, MatchAppearanceConfiguration and LedgerEntryConfiguration.

Docs only — no code touched.

🤖 Generated with Claude Code

https://claude.ai/code/session_01F9TP5WEbVsmAWQS4uAds3Q