fix: ProjectMember unique constraint, task flush before action, clean models

src/tracker/models/project.py

@@ -3,7 +3,7 @@
 import uuid
 from typing import TYPE_CHECKING
 
-from sqlalchemy import ForeignKey, Integer, String, Text
+from sqlalchemy import ForeignKey, Integer, String, Text, UniqueConstraint
 from sqlalchemy.dialects.postgresql import ARRAY, UUID
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 
@@ -32,15 +32,20 @@ class Project(Base):
 
 
 class ProjectMember(Base):
+    """Many-to-many: project ↔ member with role."""
     __tablename__ = "project_members"
 
     project_id: Mapped[uuid.UUID] = mapped_column(
-        UUID(as_uuid=True), ForeignKey("projects.id", ondelete="CASCADE"), primary_key=True
+        UUID(as_uuid=True), ForeignKey("projects.id", ondelete="CASCADE"), nullable=False
     )
     member_id: Mapped[uuid.UUID] = mapped_column(
-        UUID(as_uuid=True), ForeignKey("members.id", ondelete="CASCADE"), primary_key=True
+        UUID(as_uuid=True), ForeignKey("members.id", ondelete="CASCADE"), nullable=False
     )
     role: Mapped[str] = mapped_column(String(20), default=MemberRole.MEMBER)  # owner | member
 
     project: Mapped["Project"] = relationship(back_populates="members")
     member: Mapped["Member"] = relationship()
+
+    __table_args__ = (
+        UniqueConstraint("project_id", "member_id", name="uq_project_member"),
+    )