docs: update documentation to OKF v0.1 format

docs/database/schema.md

@@ -3,7 +3,7 @@ type: database
 title: Database Schema Overview
 description: SQLite (better-sqlite3) schema for HIPCTF: tables, relationships, indexes, and WAL journal mode.
 tags: [database, sqlite, typeorm, schema]
-timestamp: 2026-07-21T14:18:00Z
+timestamp: 2026-07-22T14:50:25Z
 ---
 
 # Overview
@@ -17,6 +17,12 @@ The schema is created by a single migration
 (`backend/src/database/migrations/1700000000000-InitSchema.ts`) and seeded
 by a second migration
 (`backend/src/database/migrations/1700000000100-SeedSystemData.ts`).
+The initial migration declares every column used by the entities
+(including `category.created_at` and `category.updated_at`), so the
+later `AddCategoryTimestampsAndUniqueAbbrev1700000000200` and
+`UpdateSystemCategoryKeys1700000000300` migrations only patch legacy
+databases that pre-date those columns and reseed the canonical system
+rows.
 
 # Tables