feat: implement email template system with vendor overrides

Add comprehensive email template management for both admin and vendors:

Admin Features:
- Email templates management page at /admin/email-templates
- Edit platform templates with language support (en, fr, de, lb)
- Preview templates with sample variables
- Send test emails
- View email logs per template

Vendor Features:
- Email templates customization page at /vendor/{code}/email-templates
- Override platform templates with vendor-specific versions
- Preview and test customized templates
- Revert to platform defaults

Technical Changes:
- Migration for vendor_email_templates table
- VendorEmailTemplate model with override management
- Enhanced EmailService with language resolution chain
  (customer preferred -> vendor preferred -> platform default)
- Branding resolution (Wizamart default, removed for whitelabel)
- Platform-only template protection (billing templates)
- Admin and vendor API endpoints with full CRUD
- Updated seed script with billing and team templates

Files: 22 changed, ~3,900 lines added

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

alembic/versions/u9c0d1e2f3g4_add_vendor_email_templates.py

@@ -0,0 +1,114 @@
+# alembic/versions/u9c0d1e2f3g4_add_vendor_email_templates.py
+"""Add vendor email templates and enhance email_templates table.
+
+Revision ID: u9c0d1e2f3g4
+Revises: t8b9c0d1e2f3
+Create Date: 2026-01-03
+
+Changes:
+- Add is_platform_only column to email_templates (templates that vendors cannot override)
+- Add required_variables column to email_templates (JSON list of required variables)
+- Create vendor_email_templates table for vendor-specific template overrides
+"""
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision = "u9c0d1e2f3g4"
+down_revision = "t8b9c0d1e2f3"
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    # Add new columns to email_templates
+    op.add_column(
+        "email_templates",
+        sa.Column("is_platform_only", sa.Boolean(), nullable=False, server_default="0"),
+    )
+    op.add_column(
+        "email_templates",
+        sa.Column("required_variables", sa.Text(), nullable=True),
+    )
+
+    # Create vendor_email_templates table
+    op.create_table(
+        "vendor_email_templates",
+        sa.Column("id", sa.Integer(), nullable=False, autoincrement=True),
+        sa.Column("vendor_id", sa.Integer(), nullable=False),
+        sa.Column("template_code", sa.String(100), nullable=False),
+        sa.Column("language", sa.String(5), nullable=False, server_default="en"),
+        sa.Column("name", sa.String(255), nullable=True),
+        sa.Column("subject", sa.String(500), nullable=False),
+        sa.Column("body_html", sa.Text(), nullable=False),
+        sa.Column("body_text", sa.Text(), nullable=True),
+        sa.Column("is_active", sa.Boolean(), nullable=False, server_default="1"),
+        sa.Column(
+            "created_at",
+            sa.DateTime(),
+            nullable=False,
+            server_default=sa.text("CURRENT_TIMESTAMP"),
+        ),
+        sa.Column(
+            "updated_at",
+            sa.DateTime(),
+            nullable=False,
+            server_default=sa.text("CURRENT_TIMESTAMP"),
+        ),
+        sa.PrimaryKeyConstraint("id"),
+        sa.ForeignKeyConstraint(
+            ["vendor_id"],
+            ["vendors.id"],
+            name="fk_vendor_email_templates_vendor_id",
+            ondelete="CASCADE",
+        ),
+        sa.UniqueConstraint(
+            "vendor_id",
+            "template_code",
+            "language",
+            name="uq_vendor_email_template_code_language",
+        ),
+    )
+
+    # Create indexes for performance
+    op.create_index(
+        "ix_vendor_email_templates_vendor_id",
+        "vendor_email_templates",
+        ["vendor_id"],
+    )
+    op.create_index(
+        "ix_vendor_email_templates_template_code",
+        "vendor_email_templates",
+        ["template_code"],
+    )
+    op.create_index(
+        "ix_vendor_email_templates_lookup",
+        "vendor_email_templates",
+        ["vendor_id", "template_code", "language"],
+    )
+
+    # Add unique constraint to email_templates for code+language
+    # This ensures we can reliably look up platform templates
+    op.create_index(
+        "ix_email_templates_code_language",
+        "email_templates",
+        ["code", "language"],
+        unique=True,
+    )
+
+
+def downgrade():
+    # Drop indexes
+    op.drop_index("ix_email_templates_code_language", table_name="email_templates")
+    op.drop_index("ix_vendor_email_templates_lookup", table_name="vendor_email_templates")
+    op.drop_index("ix_vendor_email_templates_template_code", table_name="vendor_email_templates")
+    op.drop_index("ix_vendor_email_templates_vendor_id", table_name="vendor_email_templates")
+
+    # Drop vendor_email_templates table
+    op.drop_table("vendor_email_templates")
+
+    # Remove new columns from email_templates
+    op.drop_column("email_templates", "required_variables")
+    op.drop_column("email_templates", "is_platform_only")