Seed knockout round-1 games from group-stage standings

[ylynfatt]

Summary

Phase ③ of World Cup–style advancement — stacked on #49 (entrant slot descriptors), merge that first; this PR then retargets to main automatically.

Answers "how do teams carry over from the group stage": the stage page now has a Seed from group stage card for admins.

• SeedStageFromGroups action — the stage-to-stage analog of AdvanceBracketWinner:
  • preview() resolves every entrant slot against the source stage's current standings (group positions via the per-group calculator with all in-group tiebreakers; best_placed ranks via the cross-group ranking) — read-only.
  • execute() re-resolves and fills round-1 home/away in a transaction. Idempotent: re-running after a result correction overwrites the qualifiers — refused once any round-1 game has kicked off.
• Review card on the stage page (admins only): all 16 resolved matchups shown as Team (Winner Group A) v Team (Best-placed Phase 1: foundational cleanup of teams, games, and results #1), an amber warning while the source stage still has unfinished games (seed now, re-apply later), inline errors for unresolvable slots, and a confirm-guarded Apply seeding button posting to the new stages.seed-from-groups route.
• Stage::previousGroupedStage() extracted to the model, shared by the seeder and the entrant builder's sourceStage prop.

Verified read-only against the real 2026 World Cup dev data: all 32 slots of the configured R32 resolve correctly (12 groups + 8 best-thirds).

Remaining phase ④ (separate PR): best-thirds allocation tables / no-rematch assignment + manual swap.

Testing

• 10 new feature tests: resolution correctness (both descriptor types), preview purity, idempotent re-seed after a result flip, kicked-off refusal, unresolvable-slot errors, missing-source error, page payload for owner vs guest, endpoint authorization
• Full suite: 431 passed

🤖 Generated with Claude Code