keep ingested chunks findable and structured

Two bugs made self-ingested content useless, both silently:

- _payload wrote `chapter_number` (int) while math retrieval filters on
  `chapter` (the full "فصل ۲: کسر" string), so anything we ingest could
  never match a search — no error, just permanently invisible content.
- ingest_pdf flattened the vision extraction into one text blob and threw
  away type/keywords/generalSolution/problem/solution. LessonChunk is the
  source of truth, so rebuilding meant paying for vision again — and the
  problem/solution pairs are the input for building lesson questions.

LessonChunk now stores those fields and _payload emits them under the same
keys the other team's collections use, so ours and theirs stay one shape.

Also fixes a regression from the whitenoise commit: the Manifest storage
variant raises when staticfiles.json is absent, which broke all 23 admin
rendering tests (Django forces DEBUG=False in tests, so USE_FINDERS is off)
and would have taken the admin panel down entirely the day DEBUG=0 ships
without a collectstatic run.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.gitignore

@@ -4,3 +4,7 @@ __pycache__/
 db.sqlite3
 .DS_Store
 .env
+
+# کتاب‌های درسی: خودِ PDFها در گیت نمی‌آیند (حجیم‌اند)
+data/textbooks/*
+!data/textbooks/README.md