feat: Add plugin configuration editing with template variable parsing

## Backend Changes:
- Enhanced PluginTemplate.save() to auto-parse template variables from plugin code
- Updated PluginInstallationSerializer to expose template metadata (description, category, version, author, logo, template_variables)
- Fixed template variable parser to handle nested {{ }} braces in default values
- Added brace-counting algorithm to properly extract variables with insertion codes
- Fixed explicit type parameter detection (textarea, text, email, etc.)
- Made scheduled_task optional on PluginInstallation model
- Added EventPlugin through model for event-plugin relationships
- Added Event.execute_plugins() method for plugin automation

## Frontend Changes:
- Created Tasks.tsx page for managing scheduled tasks
- Enhanced MyPlugins page with clickable plugin cards
- Added edit configuration modal with dynamic form generation
- Implemented escape sequence handling (convert \n, \', etc. for display)
- Added plugin logos to My Plugins page
- Updated type definitions for PluginInstallation interface
- Added insertion code documentation to Plugin Docs

## Plugin System:
- All platform plugins now have editable email templates with textarea support
- Template variables properly parsed with full default values
- Insertion codes ({{CUSTOMER_NAME}}, {{BUSINESS_NAME}}, etc.) documented
- Plugin logos displayed in marketplace and My Plugins

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

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

frontend/src/types.ts

@@ -314,6 +314,8 @@ export interface PluginTemplate {
   category: PluginCategory;
   version: string;
   author: string;
+  logoUrl?: string;
+  pluginCode?: string;
   rating: number;
   ratingCount: number;
   installCount: number;
@@ -330,6 +332,10 @@ export interface PluginInstallation {
   templateDescription: string;
   category: PluginCategory;
   version: string;
+  authorName?: string;
+  logoUrl?: string;
+  templateVariables?: Record<string, any>;
+  configValues?: Record<string, any>;
   isActive: boolean;
   installedAt: string;
   hasUpdate: boolean;