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/schemas.py

@@ -25,6 +25,7 @@ class ProjectCreate(BaseModel):
     name: str
     description: Optional[str] = None
     is_public: bool = True
+    team_id: Optional[UUID] = None
 
 
 class ProjectResponse(BaseModel):
@@ -35,6 +36,9 @@ class ProjectResponse(BaseModel):
     created_at: datetime
     updated_at: datetime
     created_by: str
+    team_id: Optional[UUID] = None
+    team_slug: Optional[str] = None
+    team_name: Optional[str] = None
 
     class Config:
         from_attributes = True
@@ -1053,3 +1057,139 @@ class CircularDependencyError(BaseModel):
     message: str
     cycle: List[str]  # List of "project/package" strings showing the cycle
 
+
+# Team schemas
+TEAM_ROLES = ["owner", "admin", "member"]
+RESERVED_TEAM_SLUGS = {"new", "api", "admin", "settings", "members", "projects", "search"}
+
+
+class TeamCreate(BaseModel):
+    """Create a new team"""
+    name: str
+    slug: str
+    description: Optional[str] = None
+
+    @field_validator('name')
+    @classmethod
+    def validate_name(cls, v: str) -> str:
+        """Validate team name."""
+        if not v or not v.strip():
+            raise ValueError("Name cannot be empty")
+        if len(v) > 255:
+            raise ValueError("Name must be 255 characters or less")
+        return v.strip()
+
+    @field_validator('slug')
+    @classmethod
+    def validate_slug(cls, v: str) -> str:
+        """Validate team slug format (lowercase alphanumeric with hyphens)."""
+        import re
+        if not v:
+            raise ValueError("Slug cannot be empty")
+        if len(v) < 2:
+            raise ValueError("Slug must be at least 2 characters")
+        if len(v) > 255:
+            raise ValueError("Slug must be 255 characters or less")
+        if not re.match(r'^[a-z0-9][a-z0-9-]*[a-z0-9]$', v) and not re.match(r'^[a-z0-9]$', v):
+            raise ValueError(
+                "Slug must be lowercase alphanumeric with hyphens, "
+                "starting and ending with alphanumeric characters"
+            )
+        if '--' in v:
+            raise ValueError("Slug cannot contain consecutive hyphens")
+        if v in RESERVED_TEAM_SLUGS:
+            raise ValueError(f"Slug '{v}' is reserved and cannot be used")
+        return v
+
+    @field_validator('description')
+    @classmethod
+    def validate_description(cls, v: Optional[str]) -> Optional[str]:
+        """Validate team description."""
+        if v is not None and len(v) > 2000:
+            raise ValueError("Description must be 2000 characters or less")
+        return v
+
+
+class TeamUpdate(BaseModel):
+    """Update team details"""
+    name: Optional[str] = None
+    description: Optional[str] = None
+
+    @field_validator('name')
+    @classmethod
+    def validate_name(cls, v: Optional[str]) -> Optional[str]:
+        """Validate team name."""
+        if v is not None:
+            if not v.strip():
+                raise ValueError("Name cannot be empty")
+            if len(v) > 255:
+                raise ValueError("Name must be 255 characters or less")
+            return v.strip()
+        return v
+
+    @field_validator('description')
+    @classmethod
+    def validate_description(cls, v: Optional[str]) -> Optional[str]:
+        """Validate team description."""
+        if v is not None and len(v) > 2000:
+            raise ValueError("Description must be 2000 characters or less")
+        return v
+
+
+class TeamResponse(BaseModel):
+    """Team response with basic info"""
+    id: UUID
+    name: str
+    slug: str
+    description: Optional[str]
+    created_at: datetime
+    updated_at: datetime
+    member_count: int = 0
+    project_count: int = 0
+
+    class Config:
+        from_attributes = True
+
+
+class TeamDetailResponse(TeamResponse):
+    """Team response with user's role"""
+    user_role: Optional[str] = None  # 'owner', 'admin', 'member', or None
+
+
+class TeamMemberCreate(BaseModel):
+    """Add a member to a team"""
+    username: str
+    role: str = "member"
+
+    @field_validator('role')
+    @classmethod
+    def validate_role(cls, v: str) -> str:
+        if v not in TEAM_ROLES:
+            raise ValueError(f"Role must be one of: {', '.join(TEAM_ROLES)}")
+        return v
+
+
+class TeamMemberUpdate(BaseModel):
+    """Update a team member's role"""
+    role: str
+
+    @field_validator('role')
+    @classmethod
+    def validate_role(cls, v: str) -> str:
+        if v not in TEAM_ROLES:
+            raise ValueError(f"Role must be one of: {', '.join(TEAM_ROLES)}")
+        return v
+
+
+class TeamMemberResponse(BaseModel):
+    """Team member response"""
+    id: UUID
+    user_id: UUID
+    username: str
+    email: Optional[str]
+    role: str
+    created_at: datetime
+
+    class Config:
+        from_attributes = True
+