feat: добавить отзывы, milestone-платежи, портфолио, skill-tests и верификацию (фичи Upwork/Fiverr)

Фичи конкурентов внедрены:
- Reviews API + UI — система отзывов с рейтингом 1-5 звёзд
- Milestones (Upwork-style) — разделение escrow на этапы с submit/approve
- Portfolio — портфолио фрилансера с превью работ и технологиями
- Skill Tests (Upwork-style) — сертификация навыков с тестами
- Verification Badges — верификация email/phone/id/bank

Модели: Milestone, PortfolioItem, SkillTest/SkillTestResult, Verification

backend/app/models/milestone.py

@@ -0,0 +1,26 @@
+"""Модель Milestone-платежей (Upwork-style)."""
+
+import uuid
+from datetime import datetime, timezone
+
+from sqlalchemy import Column, DateTime, Enum, Float, ForeignKey, func
+from sqlalchemy.dialects.postgresql import UUID as PG_UUID
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from app.core.database import Base
+
+
+class Milestone(Base):
+    __tablename__ = "milestones"
+
+    id: Mapped[uuid.UUID] = mapped_column(PG_UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    project_id: Mapped[uuid.UUID] = mapped_column(PG_UUID(as_uuid=True), ForeignKey("projects.id"), nullable=False)
+    escrow_transaction_id: Mapped[uuid.UUID | None] = mapped_column(PG_UUID(as_uuid=True), ForeignKey("escrow_transactions.id"))
+    title: Mapped[str] = mapped_column(String(255), nullable=False)
+    description: Mapped[str | None] = mapped_column(Text)
+    amount: Mapped[float] = mapped_column(Float(precision=10, scale=2), nullable=False)
+    status: Mapped[str] = mapped_column(Enum("pending", "funded", "in_progress", "submitted", "approved", "disputed"), default="pending")
+    due_date: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))
+    submitted_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))
+    approved_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))
+    created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), server_default=func.now())