Phase 8: Customizable PDF Templates — locale support, admin editor, seed templates

Backend:
- PdfTemplate model with locale field + UNIQUE(name, locale) constraint
- Migration 007: pdf_templates table + template_id FK on generated_pdfs
- Template service: CRUD, Jinja2 validation, render preview with sample data
- Admin endpoints: CRUD /admin/pdf-templates + POST preview
- User endpoint: GET /pdf/templates (active templates list)
- pdf_service: resolves template from DB by ID or falls back to default
  for the appropriate locale
- AI generate_pdf tool accepts optional template_id
- Seed script + 4 HTML template files:
  - Basic Report (en/ru) — general-purpose report
  - Medical Report (en/ru) — health-focused with disclaimers

Frontend:
- Admin PDF templates page with editor, locale selector, live preview (iframe),
  template variables reference panel
- PDF page: template selector dropdown in generation form
- API clients for admin CRUD + user template listing
- Sidebar: admin templates link
- English + Russian translations

Also added Phase 9 (OAuth) and Phase 10 (Rate Limits) placeholders to GeneralPlan.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/schemas/pdf.py

@@ -8,6 +8,7 @@ class GeneratePdfRequest(BaseModel):
     title: str = Field(min_length=1, max_length=255)
     document_ids: list[uuid.UUID] = []
     chat_id: uuid.UUID | None = None
+    template_id: uuid.UUID | None = None
 
 
 class PdfResponse(BaseModel):
@@ -16,6 +17,7 @@ class PdfResponse(BaseModel):
     title: str
     source_document_ids: list[uuid.UUID] | None
     source_chat_id: uuid.UUID | None
+    template_id: uuid.UUID | None
     created_at: datetime
 
     model_config = {"from_attributes": True}