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/Dockerfile

@@ -5,6 +5,9 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     gnupg \
     gzip \
     gosu \
+    tesseract-ocr \
+    tesseract-ocr-eng \
+    poppler-utils \
   && rm -rf /var/lib/apt/lists/*
 RUN pip install --no-cache-dir uv
 WORKDIR /app