refactor: complete module-driven architecture migration

This commit completes the migration to a fully module-driven architecture:

## Models Migration
- Moved all domain models from models/database/ to their respective modules:
  - tenancy: User, Admin, Vendor, Company, Platform, VendorDomain, etc.
  - cms: MediaFile, VendorTheme
  - messaging: Email, VendorEmailSettings, VendorEmailTemplate
  - core: AdminMenuConfig
- models/database/ now only contains Base and TimestampMixin (infrastructure)

## Schemas Migration
- Moved all domain schemas from models/schema/ to their respective modules:
  - tenancy: company, vendor, admin, team, vendor_domain
  - cms: media, image, vendor_theme
  - messaging: email
- models/schema/ now only contains base.py and auth.py (infrastructure)

## Routes Migration
- Moved admin routes from app/api/v1/admin/ to modules:
  - menu_config.py -> core module
  - modules.py -> tenancy module
  - module_config.py -> tenancy module
- app/api/v1/admin/ now only aggregates auto-discovered module routes

## Menu System
- Implemented module-driven menu system with MenuDiscoveryService
- Extended FrontendType enum: PLATFORM, ADMIN, VENDOR, STOREFRONT
- Added MenuItemDefinition and MenuSectionDefinition dataclasses
- Each module now defines its own menu items in definition.py
- MenuService integrates with MenuDiscoveryService for template rendering

## Documentation
- Updated docs/architecture/models-structure.md
- Updated docs/architecture/menu-management.md
- Updated architecture validation rules for new exceptions

## Architecture Validation
- Updated MOD-019 rule to allow base.py in models/schema/
- Created core module exceptions.py and schemas/ directory
- All validation errors resolved (only warnings remain)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/modules/tenancy/models/admin_platform.py

@@ -0,0 +1,164 @@
+# app/modules/tenancy/models/admin_platform.py
+"""
+AdminPlatform junction table for many-to-many relationship between Admin Users and Platforms.
+
+This enables platform-scoped admin access:
+- Super Admins: Have is_super_admin=True on User model, bypass this table
+- Platform Admins: Assigned to specific platforms via this junction table
+
+A platform admin CAN be assigned to multiple platforms (e.g., both OMS and Loyalty).
+"""
+
+from datetime import UTC, datetime
+
+from sqlalchemy import (
+    Boolean,
+    Column,
+    DateTime,
+    ForeignKey,
+    Index,
+    Integer,
+    UniqueConstraint,
+)
+from sqlalchemy.orm import relationship
+
+from app.core.database import Base
+from models.database.base import TimestampMixin
+
+
+class AdminPlatform(Base, TimestampMixin):
+    """
+    Junction table linking admin users to platforms they can manage.
+
+    Allows a platform admin to:
+    - Manage specific platforms only (not all)
+    - Be assigned to multiple platforms
+    - Have assignment tracked for audit purposes
+
+    Example:
+    - User "john@example.com" (admin) can manage OMS platform only
+    - User "jane@example.com" (admin) can manage both OMS and Loyalty platforms
+    """
+
+    __tablename__ = "admin_platforms"
+
+    id = Column(Integer, primary_key=True, index=True)
+
+    # ========================================================================
+    # Foreign Keys
+    # ========================================================================
+
+    user_id = Column(
+        Integer,
+        ForeignKey("users.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+        comment="Reference to the admin user",
+    )
+
+    platform_id = Column(
+        Integer,
+        ForeignKey("platforms.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+        comment="Reference to the platform",
+    )
+
+    # ========================================================================
+    # Assignment Status
+    # ========================================================================
+
+    is_active = Column(
+        Boolean,
+        default=True,
+        nullable=False,
+        comment="Whether the admin assignment is active",
+    )
+
+    # ========================================================================
+    # Audit Fields
+    # ========================================================================
+
+    assigned_at = Column(
+        DateTime(timezone=True),
+        default=lambda: datetime.now(UTC),
+        nullable=False,
+        comment="When the admin was assigned to this platform",
+    )
+
+    assigned_by_user_id = Column(
+        Integer,
+        ForeignKey("users.id", ondelete="SET NULL"),
+        nullable=True,
+        comment="Super admin who made this assignment",
+    )
+
+    # ========================================================================
+    # Relationships
+    # ========================================================================
+
+    user = relationship(
+        "User",
+        foreign_keys=[user_id],
+        back_populates="admin_platforms",
+    )
+
+    platform = relationship(
+        "Platform",
+        back_populates="admin_platforms",
+    )
+
+    assigned_by = relationship(
+        "User",
+        foreign_keys=[assigned_by_user_id],
+    )
+
+    # ========================================================================
+    # Constraints & Indexes
+    # ========================================================================
+
+    __table_args__ = (
+        # Each admin can only be assigned to a platform once
+        UniqueConstraint(
+            "user_id",
+            "platform_id",
+            name="uq_admin_platform",
+        ),
+        # Performance indexes
+        Index(
+            "idx_admin_platform_active",
+            "user_id",
+            "platform_id",
+            "is_active",
+        ),
+        Index(
+            "idx_admin_platform_user_active",
+            "user_id",
+            "is_active",
+        ),
+    )
+
+    # ========================================================================
+    # Properties
+    # ========================================================================
+
+    @property
+    def platform_code(self) -> str | None:
+        """Get the platform code for this assignment."""
+        return self.platform.code if self.platform else None
+
+    @property
+    def platform_name(self) -> str | None:
+        """Get the platform name for this assignment."""
+        return self.platform.name if self.platform else None
+
+    def __repr__(self) -> str:
+        return (
+            f"<AdminPlatform("
+            f"user_id={self.user_id}, "
+            f"platform_id={self.platform_id}, "
+            f"is_active={self.is_active})>"
+        )
+
+
+__all__ = ["AdminPlatform"]