Model the whole domain: communities, sessions, ratings and operations

[elgorro]

Lays down the data model the product needs, replacing a schema that was players,
teams, matches and tournaments — with an EloRating class nothing outside the
tests ever called.

Refs #32.

What changes

45 entities across eleven bounded contexts, one EF configuration per entity, and
five migrations that each apply and revert on their own.

Three changes are structural:

• Rating moves from Player to CommunityMember. A player’s standing is
  relative to the people they play with, not global.
• MatchAppearance records who actually played, with the rating before,
  after and delta. Teams are lineups and change; who played a given match does
  not. This is what makes ratings explainable and replayable.
• Matches snapshot the scoring they were played under, so amending a rule
  set cannot rewrite the past.

Elo is no longer dead code. MatchRatingCalculator sits on top of
EloRating, and MatchQueries.CreateAsync applies ratings on entry. Community
and format are derived from the teams — a team already belongs to a community,
and its size is the format — so the WebAssembly client needed no new fields.

Money is double-entry in minor units, which makes dues, session fees, expenses
and sponsorship one mechanism rather than four.

Enforced by the database

Application-level checks race, so these are constraints:

• Court reservations cannot overlap (EXCLUDE USING gist, half-open ranges)
• Ledger amounts are positive and move between two different accounts
• A member-balance account has a member; nothing else does
• An item is on loan to at most one person at a time
• A community has at most one current season

Verification

Each migration was applied against a throwaway Postgres 18, then the whole chain
reverted (46 tables back to 7) and rolled forward again.

• Slug backfill — collisions suffixed (jonas-mueller, jonas-mueller-2),
  German transliterated (strassburger-weiss), degenerate names fall back to player
• Ratings survive the move in both directions — Down() carries them back
  before the memberships holding them are dropped
• Court overlap — five cases: overlapping rejected, abutting slots allowed,
  cancelled ignored, inverted range rejected
• Money — negative amounts, same-account entries and member-balance accounts
  without a member all rejected
• Appearances reconstructed from existing lineups, two a side, ratings left
  flat because the deltas that produced them were never recorded

22 tests pass. Release build clean, dotnet format --verify-no-changes clean.

Worth knowing when reviewing

1. The Location backfill is a no-op in practice. The intent was to match
   free text against courts, but no courts exist at that point in the chain, so
   it all lands in LocationNote. EF detected a column rename, so nothing was
   lost. Turning those strings into venues would have meant fabricating data.

2. TeamMember kept its composite key. A surrogate Id was planned;
   nothing needs one, and adding it means a primary key rebuild for no gain.
   Position and SortOrder were added as intended.

3. The rating write path has no integration test. The calculator is well
   covered and the schema is verified, but the glue in CreateAsync is not —
   there is no infrastructure test project. See Integration test for the rating write path #34.

4. Entities are grouped into folders by context but keep the flat
   Ssabba.Domain.Entities namespace, so no existing using churned.
   IDE0130 is silenced in .editorconfig with a note.

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