feat: Player Enrollment — DM targets 4-6 Discord users as player characters

[Fortinbra]

The DM targets 4-6 Discord users to become player characters in the active campaign. Invitation/acceptance flow. Links Discord users to PlayerCharacter entities in the world.

[Fortinbra]

✅ Spec approved by The Doctor. Ready for Danny's TDD kickoff ceremony.

Review findings:

• ✓ CampaignPlayer.CampaignId FK to Campaign.Id (spec feat: Campaign Initialization — Discord channel binding and management UI override #7 dependency)
• ✓ CampaignPlayer.PlayerCharacterId FK to PlayerCharacter.Id (spec feat: World Building — configure world lore and settings for the AI Dungeon Master #4 dependency)
• ✓ 4-6 player constraint enforced in domain logic (soft warning, hard limit at 6)
• ✓ Discord button interaction security correctly validates user identity
• ✓ Unique constraint prevents duplicate invites (same user + campaign)

Watch point: Danny must test Discord button interaction security (only invited user can accept their own invite)

Dependencies: Spec #4 (PlayerCharacter), Spec #7 (Campaign)

Agent assignment: Rory (domain entities, migrations, API endpoints, Discord commands with button interactions), Clara (UI for player enrollment list in campaign detail page)

[Fortinbra]

Spec #8 Player Enrollment verified by Danny — all tests passing. ✅

Test results:

• Core PlayerEnrollment: 8/8 passing
• API PlayerEnrollment: 5/5 passing
• Total spec feat: Player Enrollment — DM targets 4-6 Discord users as player characters #8 tests: 13/13 passing
• Full suite (excluding pre-existing JWT config failures): 264 passing