docs: add migration system documentation and generated initial migrations

Added `README.md` to document the database migration system, including generation and usage instructions. Updated `generate-migrations.ts` to fix directory structure and streamline commands. Included initial migration files for schema setup using DrizzleORM.

backend/src/database/migrations/sql/meta/_journal.json

@@ -0,0 +1,13 @@
+{
+  "version": "6",
+  "dialect": "postgresql",
+  "entries": [
+    {
+      "idx": 0,
+      "version": "6",
+      "when": 1747322785586,
+      "tag": "0000_lively_tiger_shark",
+      "breakpoints": true
+    }
+  ]
+}