Add lessons Phase 4: lesson sessions with hybrid calendar generation

Individual lesson occurrences generated from schedule slot patterns.
Idempotent session generation with configurable rolling window.
Post-lesson notes workflow with auto-set notesCompletedAt. Status
tracking (scheduled/attended/missed/makeup/cancelled) and date/time
filtering. 13 new tests (64 total lessons tests).

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

@@ -218,6 +218,13 @@
       "when": 1774900000000,
       "tag": "0030_enrollments",
       "breakpoints": true
+    },
+    {
+      "idx": 31,
+      "version": "7",
+      "when": 1774910000000,
+      "tag": "0031_lesson_sessions",
+      "breakpoints": true
     }
   ]
 }