Implement product selection landing page (Feature 002)

Adds landing page at root URL (/) that displays all active products with links to feedback submission forms. This replaces the requirement for users to know direct product URLs.

Changes:
- Added Product.load_active() method to filter and sort active products alphabetically
- Created landing route blueprint with error handling and structured logging
- Registered landing blueprint in app factory, replacing old index route
- Created landing page template with product list and empty state
- Added comprehensive contract tests (6 tests) covering active products, filtering, sorting, XSS prevention
- Added integration test for complete user flow from landing page to submission form

All 7 tests pass. User Story 1 (P1 - MVP) complete.

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

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

app/__init__.py

@@ -182,18 +182,12 @@ def create_app(config_name='development'):
     )
 
     # Register blueprints
-    from app.routes import submission, dashboard, admin, auth
+    from app.routes import submission, dashboard, admin, auth, landing
     app.register_blueprint(submission.bp)
     app.register_blueprint(dashboard.bp)
     app.register_blueprint(admin.bp)
     app.register_blueprint(auth.bp)
-
-    # Set index route
-    @app.route('/')
-    def index():
-        """Welcome page"""
-        from flask import render_template
-        return render_template('index.html')
+    app.register_blueprint(landing.landing_bp)  # Landing page (product selection)
 
     # Health check endpoint (T208)
     @app.route('/health')