Add accounts, members, and processor-agnostic payment linking

- account table (billing entity, soft-delete, company-scoped)
- member table (people on an account, is_minor from DOB)
- account_processor_link table (maps accounts to any payment
  processor — stripe, global_payments — instead of stripe_customer_id
  directly on account)
- Full CRUD routes + search (name, email, phone, account_number)
- Member routes nested under accounts with isMinor auto-calculation
- Zod validation schemas in @forte/shared
- 19 tests passing

packages/backend/src/db/migrations/meta/_journal.json

@@ -22,6 +22,13 @@
       "when": 1774648659531,
       "tag": "0002_bumpy_mandarin",
       "breakpoints": true
+    },
+    {
+      "idx": 3,
+      "version": "7",
+      "when": 1774651222033,
+      "tag": "0003_round_captain_midlands",
+      "breakpoints": true
     }
   ]
 }