backend: prescription start/end dates + dispense ledger

Add optional startDate/endDate columns to prescriptions (schema,
validation, types, service) — when set, endDate drives expiry.

Add a new append-only `dispenses` resource (schema, types, validation,
service, route at /api/dispenses) recording who received which medication,
gated on the existing inventory RBAC statement. Migration 0020.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/drizzle/meta/_journal.json

@@ -141,6 +141,13 @@
       "when": 1781538797034,
       "tag": "0019_black_mole_man",
       "breakpoints": true
+    },
+    {
+      "idx": 20,
+      "version": "7",
+      "when": 1781629630102,
+      "tag": "0020_freezing_tomas",
+      "breakpoints": true
     }
   ]
 }