chore(matrix): scrub final matrix references after subsystem removal

The matrix subsystem was removed in a prior commit. This commit cleans
up the small leftovers that grep found:

- docker-compose.macmini.yml: dropped the "Matrix Stack" port-range
  comment, the "matrix" category from the naming convention, and a
  stale watchtower comment about Matrix notifications.
- packages/credits/src/operations.ts: removed AI_BOT_CHAT credit
  operation type and its definition. It was the billing entry for "Chat
  with AI via Matrix bot" — no callers left.
- services/mana-credits gifts schema + service + validation: removed the
  targetMatrixId column / param / Zod field. The corresponding
  PostgreSQL column was dropped manually with
  `ALTER TABLE gifts.gift_codes DROP COLUMN target_matrix_id` on prod.
- docker/grafana/dashboards/{master,system}-overview.json: removed the
  `up{job="synapse"}` panel queries — they would have shown No Data
  forever now that Synapse is gone.

Production-side cleanup performed in parallel (not in this commit):
- Stopped + removed mana-matrix-{synapse,element,web,bot} containers
- Removed mana-matrix-bot:local, matrix-web:latest,
  matrixdotorg/synapse:latest, vectorim/element-web:latest images (~3 GB)
- Removed mana-matrix-bots-data Docker volume
- Removed /Volumes/ManaData/matrix/ media store (4.3 MB)
- DROP DATABASE matrix; DROP DATABASE synapse; on Postgres

Cosmetic leftovers intentionally untouched:
- Eisenhower matrix in todo (LayoutMode 'matrix') — productivity concept
- ${{ matrix.service }} in .github/workflows — GitHub Actions strategy
- services/mana-media/apps/api/dist/.../matrix/* — stale build output
  (not in git, regenerated next mana-media build)

services/mana-credits/src/db/schema/gifts.ts

@@ -61,7 +61,6 @@ export const giftCodes = giftsSchema.table(
 
 		// Personalization
 		targetEmail: text('target_email'),
-		targetMatrixId: text('target_matrix_id'),
 
 		// Riddle
 		riddleQuestion: text('riddle_question'),

services/mana-credits/src/lib/validation.ts

@@ -48,7 +48,6 @@ export const createGiftSchema = z.object({
 	type: z.enum(['simple', 'personalized', 'split', 'first_come', 'riddle']).default('simple'),
 	totalPortions: z.number().int().positive().max(100).default(1),
 	targetEmail: z.string().email().optional(),
-	targetMatrixId: z.string().optional(),
 	riddleQuestion: z.string().max(200).optional(),
 	riddleAnswer: z.string().optional(),
 	message: z.string().max(500).optional(),

services/mana-credits/src/services/gift-code.ts

@@ -22,7 +22,6 @@ interface CreateGiftParams {
 	type?: 'simple' | 'personalized' | 'split' | 'first_come' | 'riddle';
 	totalPortions?: number;
 	targetEmail?: string;
-	targetMatrixId?: string;
 	riddleQuestion?: string;
 	riddleAnswer?: string;
 	message?: string;
@@ -118,7 +117,6 @@ export class GiftCodeService {
 					totalPortions,
 					type,
 					targetEmail: params.targetEmail,
-					targetMatrixId: params.targetMatrixId,
 					riddleQuestion: params.riddleQuestion,
 					riddleAnswerHash,
 					message: params.message,