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/organization.py

@@ -0,0 +1,31 @@
+# app/models/organization.py
+from sqlalchemy import Column, String, Boolean, Text, Index
+from sqlalchemy.dialects.postgresql import JSONB
+from sqlalchemy.orm import relationship
+
+from .base import Base, TimestampMixin, UUIDMixin
+
+
+class Organization(Base, UUIDMixin, TimestampMixin):
+    """
+    Organization model for multi-tenant support.
+    Users can belong to multiple organizations with different roles.
+    """
+    __tablename__ = 'organizations'
+
+    name = Column(String(255), nullable=False, index=True)
+    slug = Column(String(255), unique=True, nullable=False, index=True)
+    description = Column(Text, nullable=True)
+    is_active = Column(Boolean, default=True, nullable=False, index=True)
+    settings = Column(JSONB, default={})
+
+    # Relationships
+    user_organizations = relationship("UserOrganization", back_populates="organization", cascade="all, delete-orphan")
+
+    __table_args__ = (
+        Index('ix_organizations_name_active', 'name', 'is_active'),
+        Index('ix_organizations_slug_active', 'slug', 'is_active'),
+    )
+
+    def __repr__(self):
+        return f"<Organization {self.name} ({self.slug})>"