Add paperless-ngx integration for document storage and share links

- dlib/integrations/paperless.py: sync HTTP client wrapping the paperless-ngx
  REST API (upload doc, poll task, create/delete share links, delete document)
- config: PAPERLESS_ENABLED, PAPERLESS_BASE_URL, PAPERLESS_TOKEN, PAPERLESS_TAG_IDS
- PublicAsset model: paperless_document_id + paperless_share_slug columns
- publication service: after creating the asset, if paperless is enabled upload
  the patched PDF and create a share link; stores doc id + share slug on the asset
- public routes: pass expires_at through to publish_version; new
  POST /{slug}/share-links endpoint to (re)create expiring share links on demand
- schemas: PublishRequest.expires_at, PublicAssetResponse.paperless_share_url,
  new ShareLinkRequest model
- frontend: paperless_share_url field on PublicAsset type, createShareLink()
  and expiresAt param on publishVersion() in api.ts
- .env.example: documented paperless env vars

https://claude.ai/code/session_01YPVs6uBwCvcwVMvrfLBBdu

apps/backend/fastapi/app/models/cv.py

@@ -3,7 +3,7 @@ from __future__ import annotations
 import enum
 from datetime import datetime, timezone
 
-from sqlalchemy import Boolean, DateTime, Enum, ForeignKey, String, Text
+from sqlalchemy import Boolean, DateTime, Enum, ForeignKey, Integer, String, Text
 from sqlalchemy.dialects.postgresql import JSONB
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 
@@ -148,6 +148,8 @@ class PublicAsset(Base, IdentifierMixin, TimestampMixin):
     expires_at: Mapped[str | None] = mapped_column(
         DateTime(timezone=True), nullable=True
     )
+    paperless_document_id: Mapped[int | None] = mapped_column(Integer, nullable=True)
+    paperless_share_slug: Mapped[str | None] = mapped_column(String(160), nullable=True)
 
     submission: Mapped[Submission | None] = relationship(
         "Submission", back_populates="public_asset"