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)

docker/grafana/dashboards/master-overview.json

@@ -510,12 +510,6 @@
 					"legendFormat": "Media",
 					"refId": "T"
 				},
-				{
-					"datasource": { "type": "prometheus", "uid": "${datasource}" },
-					"expr": "up{job=\"synapse\"}",
-					"legendFormat": "Synapse",
-					"refId": "U"
-				},
 				{
 					"datasource": { "type": "prometheus", "uid": "${datasource}" },
 					"expr": "pg_up",

docker/grafana/dashboards/system-overview.json

@@ -637,12 +637,6 @@
 					"legendFormat": "Media",
 					"refId": "T"
 				},
-				{
-					"datasource": { "type": "prometheus", "uid": "${datasource}" },
-					"expr": "up{job=\"synapse\"}",
-					"legendFormat": "Synapse",
-					"refId": "U"
-				},
 				{
 					"datasource": { "type": "prometheus", "uid": "${datasource}" },
 					"expr": "pg_up",