Phase 3: Skills & Context — skill system, personal context, context layering

Backend:
- Skill model + migration (with FK on chats.skill_id)
- Personal + general skill CRUD services with access isolation
- Admin skill CRUD endpoints (POST/GET/PATCH/DELETE /admin/skills)
- User skill CRUD endpoints (POST/GET/PATCH/DELETE /skills/)
- Personal context GET/PUT at /users/me/context
- Extended context assembly: primary + personal context + skill prompt
- Chat creation/update now accepts skill_id with validation

Frontend:
- Skill selector dropdown in chat header (grouped: general + personal)
- Reusable skill editor form component
- Admin skills management page (/admin/skills)
- Personal skills page (/skills)
- Personal context editor page (/profile/context)
- Updated sidebar: Skills, My Context nav items + admin skills link
- English + Russian translations for all skill/context UI

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

backend/app/models/user.py

@@ -26,3 +26,4 @@ class User(Base):
 
     sessions: Mapped[list["Session"]] = relationship(back_populates="user", cascade="all, delete-orphan")  # noqa: F821
     chats: Mapped[list["Chat"]] = relationship(back_populates="user", cascade="all, delete-orphan")  # noqa: F821
+    skills: Mapped[list["Skill"]] = relationship(back_populates="user", cascade="all, delete-orphan")  # noqa: F821