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/skill.py

@@ -0,0 +1,23 @@
+import uuid
+
+from sqlalchemy import Boolean, ForeignKey, Integer, String, Text
+from sqlalchemy.dialects.postgresql import UUID
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from app.database import Base
+
+
+class Skill(Base):
+    __tablename__ = "skills"
+
+    user_id: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("users.id", ondelete="CASCADE"), nullable=True, index=True
+    )
+    name: Mapped[str] = mapped_column(String(100), nullable=False)
+    description: Mapped[str | None] = mapped_column(Text, nullable=True)
+    system_prompt: Mapped[str] = mapped_column(Text, nullable=False)
+    icon: Mapped[str | None] = mapped_column(String(50), nullable=True)
+    is_active: Mapped[bool] = mapped_column(Boolean, nullable=False, default=True)
+    sort_order: Mapped[int] = mapped_column(Integer, nullable=False, default=0)
+
+    user: Mapped["User | None"] = relationship(back_populates="skills")  # noqa: F821