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/chat.py

@@ -15,11 +15,14 @@ class Chat(Base):
         UUID(as_uuid=True), ForeignKey("users.id", ondelete="CASCADE"), nullable=False, index=True
     )
     title: Mapped[str] = mapped_column(String(255), nullable=False, default="New Chat")
-    skill_id: Mapped[uuid.UUID | None] = mapped_column(UUID(as_uuid=True), nullable=True)
+    skill_id: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("skills.id", ondelete="SET NULL"), nullable=True
+    )
     is_archived: Mapped[bool] = mapped_column(Boolean, nullable=False, default=False)
     updated_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), server_default=func.now(), onupdate=func.now(), nullable=False
     )
 
     user: Mapped["User"] = relationship(back_populates="chats")  # noqa: F821
+    skill: Mapped["Skill | None"] = relationship()  # noqa: F821
     messages: Mapped[list["Message"]] = relationship(back_populates="chat", cascade="all, delete-orphan")  # noqa: F821