Add multi-tenancy with Teams feature

Implement team-based organization for projects with role-based access control:

Backend:
- Add teams and team_memberships database tables (migrations 009, 009b)
- Add Team and TeamMembership ORM models with relationships
- Implement TeamAuthorizationService for team-level access control
- Add team CRUD, membership, and projects API endpoints
- Update project creation to support team assignment

Frontend:
- Add TeamContext for managing team state with localStorage persistence
- Add TeamSelector component for switching between teams
- Add TeamsPage, TeamDashboardPage, TeamSettingsPage, TeamMembersPage
- Add team API client functions
- Update navigation with Teams link

Security:
- Team role hierarchy: owner > admin > member
- Membership checked before system admin fallback
- Self-modification prevention for role changes
- Email visibility restricted to team admins/owners
- Slug validation rejects consecutive hyphens

Tests:
- Unit tests for TeamAuthorizationService
- Integration tests for all team API endpoints

backend/app/models.py

@@ -32,6 +32,7 @@ class Project(Base):
         DateTime(timezone=True), default=datetime.utcnow, onupdate=datetime.utcnow
     )
     created_by = Column(String(255), nullable=False)
+    team_id = Column(UUID(as_uuid=True), ForeignKey("teams.id", ondelete="SET NULL"))
 
     packages = relationship(
         "Package", back_populates="project", cascade="all, delete-orphan"
@@ -39,10 +40,12 @@ class Project(Base):
     permissions = relationship(
         "AccessPermission", back_populates="project", cascade="all, delete-orphan"
     )
+    team = relationship("Team", back_populates="projects")
 
     __table_args__ = (
         Index("idx_projects_name", "name"),
         Index("idx_projects_created_by", "created_by"),
+        Index("idx_projects_team_id", "team_id"),
     )
 
 
@@ -369,6 +372,9 @@ class User(Base):
     sessions = relationship(
         "Session", back_populates="user", cascade="all, delete-orphan"
     )
+    team_memberships = relationship(
+        "TeamMembership", back_populates="user", cascade="all, delete-orphan"
+    )
 
     __table_args__ = (
         Index("idx_users_username", "username"),
@@ -561,3 +567,73 @@ class ArtifactDependency(Base):
             unique=True,
         ),
     )
+
+
+class Team(Base):
+    """Team for organizing projects and users."""
+
+    __tablename__ = "teams"
+
+    id = Column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    name = Column(String(255), nullable=False)
+    slug = Column(String(255), unique=True, nullable=False)
+    description = Column(Text)
+    created_at = Column(DateTime(timezone=True), default=datetime.utcnow)
+    updated_at = Column(
+        DateTime(timezone=True), default=datetime.utcnow, onupdate=datetime.utcnow
+    )
+    created_by = Column(String(255), nullable=False)
+    settings = Column(JSON, default=dict)
+
+    # Relationships
+    memberships = relationship(
+        "TeamMembership", back_populates="team", cascade="all, delete-orphan"
+    )
+    projects = relationship("Project", back_populates="team")
+
+    __table_args__ = (
+        Index("idx_teams_slug", "slug"),
+        Index("idx_teams_created_by", "created_by"),
+        Index("idx_teams_created_at", "created_at"),
+        CheckConstraint(
+            "slug ~ '^[a-z0-9][a-z0-9-]*[a-z0-9]$' OR slug ~ '^[a-z0-9]$'",
+            name="check_team_slug_format",
+        ),
+    )
+
+
+class TeamMembership(Base):
+    """Maps users to teams with their roles."""
+
+    __tablename__ = "team_memberships"
+
+    id = Column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    team_id = Column(
+        UUID(as_uuid=True),
+        ForeignKey("teams.id", ondelete="CASCADE"),
+        nullable=False,
+    )
+    user_id = Column(
+        UUID(as_uuid=True),
+        ForeignKey("users.id", ondelete="CASCADE"),
+        nullable=False,
+    )
+    role = Column(String(20), nullable=False, default="member")
+    created_at = Column(DateTime(timezone=True), default=datetime.utcnow)
+    invited_by = Column(String(255))
+
+    # Relationships
+    team = relationship("Team", back_populates="memberships")
+    user = relationship("User", back_populates="team_memberships")
+
+    __table_args__ = (
+        Index("idx_team_memberships_team_id", "team_id"),
+        Index("idx_team_memberships_user_id", "user_id"),
+        Index("idx_team_memberships_role", "role"),
+        Index("idx_team_memberships_team_role", "team_id", "role"),
+        Index("idx_team_memberships_unique", "team_id", "user_id", unique=True),
+        CheckConstraint(
+            "role IN ('owner', 'admin', 'member')",
+            name="check_team_role",
+        ),
+    )