Add AI receipt scanning with OCR pipeline and debug toggle

- OCR pipeline: Tesseract (images) + pdfplumber (PDFs) → AI text prompt →
  rule-based regex fallback; works with any text model, not just vision models
- Scan Receipt toolbar button parses a photo and pre-fills the transaction form;
  receipt image is automatically attached to the created transaction
- AI settings page: provider, API key (AES-256-GCM encrypted), custom URL,
  model, and per-user debug toggle that gates the OCR/AI debug panel
- Fix CSRF cookie secure=False so HTTP deployments work; add 7-day max_age
- Fix attachment_refs missing from _to_response (attachments never appeared in UI)
- Fix multipart boundary lost when Content-Type was set manually in axios calls
- nginx: raise client_max_body_size to 15 MB, add 120s proxy timeout for OCR
- Migration 0005: add ai_debug boolean to users table
- Update README and CLAUDE.md with AI scanning docs and architecture notes

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

backend/pyproject.toml

@@ -31,6 +31,9 @@ dependencies = [
     "structlog>=24.0",
     "pillow>=11.0",
     "python-magic>=0.4",
+    "pytesseract>=0.3",
+    "pdfplumber>=0.11",
+    "pdf2image>=1.17",
     "psycopg2-binary>=2.9",
 ]