feat(creator): add key deprecation and buyback endpoint with holder notifications

[TS-mfon]

Summary

Implements the creator key deprecation endpoint, enabling key creators to initiate an orderly buyback wind-down with solvency verification, on-chain deprecation contract execution, trading pause, and asynchronous notification dispatch to all key holders.

Closes #872

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Implementation Details

• Endpoint: POST /api/v1/creator/:keyId/deprecate
• Authentication & Validation:
  • Enforces caller is the creator of the key (requireKeyCreator('keyId')).
  • Enforces buybackPricePerKey is a positive integer > 0 (returns 422 for 0, negative, or non-integer).
  • Checks creator's XLM wallet balance to verify solvency for the full buyback obligation (circulatingSupply * buybackPricePerKey). Returns 400 INSUFFICIENT_BALANCE if underfunded.
• Contract Dispatch & Lifecycle:
  • Dispatches deprecate_key contract call via deprecateGateway.deprecateKey.
  • Updates CreatorProfile.status = 'Deprecated' and sets tradingPaused = true.
  • Records KEY_DEPRECATED entry in Activity table.
• Holder Notification Dispatch:
  • Queries all current holders with non-zero balance (balance > 0).
  • Uses getRedisClient() to enqueue structured notification events to queue:notifications.
• Database Migration:
  • Added migration SQL in prisma/migrations/20260924_add_key_deprecation/migration.sql for status column and KEY_DEPRECATED enum.

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Testing

• Unit test suite added in src/modules/creator/__tests__/creator-deprecate.test.ts verifying:
  • 401 on unauthenticated calls.
  • 403 when caller is not the creator of the key.
  • 422 when buyback price is 0, negative, or non-integer.
  • 400 when creator has insufficient XLM balance.
  • 200 on valid requests: executes contract call, updates status to Deprecated, sets tradingPaused, creates activity record, and enqueues notifications to Redis for each holder.
• All 6 unit tests pass; TypeScript build passes cleanly.

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[TS-mfon]

Superseded by #1007, which carries all four issues (#866, #869, #870, #872) on one branch rebased on latest main.

Splitting these four across separate PRs meant they collided with each other: all four touch src/modules/keys/keys.routes.ts, and #869/#872 both touch src/modules/creator/creator.routes.ts and the prisma schema, so whichever merged first would break the rest. #1007 also addresses the Copilot findings raised here — the migration moved to prisma/schema/migrations and non-finite milestone values are now rejected.

Closing this one in favour of #1007.