feat(ocr-review): replace LLM with rule-based spell-checker (REVIEW_ENGINE=spell)

- Add pyspellchecker (MIT) to requirements for EN+DE dictionary lookup
- New spell_review_entries_sync() + spell_review_entries_streaming():
  - Dictionary-backed substitution: checks if corrected word is known
  - Structural rule: digit at pos 0 + lowercase rest → most likely letter
    (e.g. "8en"→"Ben", "8uch"→"Buch", "5ee"→"See", "6eld"→"Geld")
  - Pattern rule: "|." → "1." for numbered list prefixes
  - Standalone "|" → "I" (capital I)
  - IPA entries still protected via existing _entry_needs_review filter
  - Headings/untranslated words (e.g. "Story") are untouched (no susp. chars)
- llm_review_entries + llm_review_entries_streaming: route via REVIEW_ENGINE
  env var ("spell" default, "llm" to restore previous behaviour)
- docker-compose.yml: REVIEW_ENGINE=${REVIEW_ENGINE:-spell}
- LLM code preserved for fallback (set REVIEW_ENGINE=llm in .env)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

klausur-service/backend/requirements.txt

@@ -35,6 +35,9 @@ onnxruntime
 # IPA pronunciation dictionary lookup (MIT license, bundled CMU dict ~134k words)
 eng-to-ipa
 
+# Spell-checker for rule-based OCR correction (MIT license)
+pyspellchecker>=0.8.1
+
 # PostgreSQL (for metrics storage)
 psycopg2-binary>=2.9.0
 asyncpg>=0.29.0