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/app/db/models/user.py

@@ -33,6 +33,7 @@ class User(Base):
     ai_api_key_enc: Mapped[bytes | None] = mapped_column(LargeBinary, nullable=True)
     ai_base_url: Mapped[str | None] = mapped_column(Text, nullable=True)
     ai_model: Mapped[str | None] = mapped_column(Text, nullable=True)
+    ai_debug: Mapped[bool] = mapped_column(Boolean, default=False, nullable=False)
 
     accounts: Mapped[list["Account"]] = relationship(back_populates="user", lazy="noload")  # type: ignore[name-defined]
     sessions: Mapped[list["Session"]] = relationship(back_populates="user", lazy="noload")  # type: ignore[name-defined]