Add organization management and admin-specific APIs

- Introduced schemas for organizations, including creation, updates, and responses.
- Created models for `Organization` and `UserOrganization` with role-based access control and relationships.
- Implemented admin APIs for managing users, organizations, and bulk actions.
- Added advanced filtering, sorting, and pagination for user and organization queries.
- Updated `CRUD` logic to support organization-specific operations and member management.
- Enhanced database with necessary indexes and validation for improved performance and data integrity.

backend/app/models/user_organization.py

@@ -0,0 +1,50 @@
+# app/models/user_organization.py
+from enum import Enum as PyEnum
+
+from sqlalchemy import Column, ForeignKey, Boolean, String, Index, Enum
+from sqlalchemy.dialects.postgresql import UUID as PGUUID
+from sqlalchemy.orm import relationship
+
+from .base import Base, TimestampMixin
+
+
+class OrganizationRole(str, PyEnum):
+    """
+    Built-in organization roles.
+    These provide a baseline role system that can be optionally used.
+    Projects can extend this or implement their own permission system.
+    """
+    OWNER = "owner"  # Full control over organization
+    ADMIN = "admin"  # Can manage users and settings
+    MEMBER = "member"  # Regular member with standard access
+    GUEST = "guest"  # Limited read-only access
+
+
+class UserOrganization(Base, TimestampMixin):
+    """
+    Junction table for many-to-many relationship between Users and Organizations.
+    Includes role information for flexible RBAC.
+    """
+    __tablename__ = 'user_organizations'
+
+    user_id = Column(PGUUID(as_uuid=True), ForeignKey('users.id', ondelete='CASCADE'), primary_key=True)
+    organization_id = Column(PGUUID(as_uuid=True), ForeignKey('organizations.id', ondelete='CASCADE'), primary_key=True)
+
+    role = Column(Enum(OrganizationRole), default=OrganizationRole.MEMBER, nullable=False, index=True)
+    is_active = Column(Boolean, default=True, nullable=False, index=True)
+
+    # Optional: Custom permissions override for specific users
+    custom_permissions = Column(String(500), nullable=True)  # JSON array of permission strings
+
+    # Relationships
+    user = relationship("User", back_populates="user_organizations")
+    organization = relationship("Organization", back_populates="user_organizations")
+
+    __table_args__ = (
+        Index('ix_user_org_user_active', 'user_id', 'is_active'),
+        Index('ix_user_org_org_active', 'organization_id', 'is_active'),
+        Index('ix_user_org_role', 'role'),
+    )
+
+    def __repr__(self):
+        return f"<UserOrganization user={self.user_id} org={self.organization_id} role={self.role}>"