feat(02-12.1): stamp a record-context epoch on every session

- add sessions.record_epoch, owned by a BEFORE UPDATE trigger so the
  ON DELETE SET NULL path and raw SQL writes cannot skip it
- normalise pre-existing rows so epoch zero means never switched
- project the epoch off the row getSession already loads

src/app/api/insights/targets/__tests__/route.test.ts

@@ -100,6 +100,7 @@ const SESSION_OK = {
     id: "sess-1",
     expiresAt: new Date(Date.now() + 3_600_000),
     actingAsUserId: null,
+    recordEpoch: 0,
   },
   user: SESSION_USER as never,
 };

src/app/api/mcp/oauth/__tests__/authorize.test.ts

@@ -81,6 +81,7 @@ function signedIn() {
       id: "s1",
       expiresAt: new Date(Date.now() + 1e6),
       actingAsUserId: null,
+      recordEpoch: 0,
     },
     user: { id: "user-1" } as never,
   });