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/models/pdf_template.py

@@ -0,0 +1,16 @@
+from sqlalchemy import Boolean, String, Text, UniqueConstraint
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.database import Base
+
+
+class PdfTemplate(Base):
+    __tablename__ = "pdf_templates"
+    __table_args__ = (UniqueConstraint("name", "locale", name="uq_pdf_templates_name_locale"),)
+
+    name: Mapped[str] = mapped_column(String(100), nullable=False)
+    locale: Mapped[str] = mapped_column(String(10), nullable=False, default="en")
+    description: Mapped[str | None] = mapped_column(Text, nullable=True)
+    html_content: Mapped[str] = mapped_column(Text, nullable=False)
+    is_default: Mapped[bool] = mapped_column(Boolean, nullable=False, default=False)
+    is_active: Mapped[bool] = mapped_column(Boolean, nullable=False, default=True)