fix(maintenance): auto-expiry + adaptive markdown height + tests

- Auto-expiry: expired events auto-end on GET /events via task manager
- Height: _estimate_markdown_height adapted to unit=8px scale with padding detection
- CRITICAL-1: removed dead import remove_chart_from_position (QA finding)
- CRITICAL-2: added chart alive check in ensure_banner_chart (QA finding)
- CRITICAL-3: fixed test assertions update_markdown_chart → update_banner_on_dashboard
- @POST contract: fixed return range [2,12] → [19,200]
- Tests: 8 new tests (auto-expiry + layout height)

frontend/src/lib/i18n/locales/en/settings.json

@@ -119,5 +119,29 @@
   "server_url": "Server URL",
   "personal_access_token": "Personal Access Token (PAT)",
   "default_repository_optional": "Default Repository (Optional)",
-  "save_configuration": "Save Configuration"
+  "save_configuration": "Save Configuration",
+
+  "system": "System",
+  "system_description": "Application-wide system settings including timezone.",
+  "app_timezone": "Timezone",
+  "app_timezone_hint": "All schedule times and timestamps will be displayed and executed in this timezone.",
+  "save_system": "Save system settings",
+  "help_git_display_name": "A friendly name to identify this Git server configuration.",
+  "help_git_provider": "Git provider type: GitHub, GitLab, or Gitea (self-hosted).",
+  "help_git_server_url": "Base URL of the Git server. For GitHub: https://github.com. For self-hosted GitLab/Gitea: your instance URL.",
+  "help_git_pat": "Personal Access Token with repository access permissions. Used for authentication with the Git server.",
+  "help_git_default_repo": "Default repository in 'owner/repo' format. Optional — can be overridden per dashboard.",
+  "help_git_default_branch": "Default branch for new commit operations. Usually 'main' or 'master'.",
+  "help_llm_chatbot_provider": "LLM provider used by the AI assistant for intent planning and conversation routing.",
+  "help_llm_chatbot_model": "Optional model name override. Leave empty to use the provider's default model. Example: gpt-4.1-mini.",
+  "help_llm_binding_dashboard_validation": "Provider used for automated dashboard validation. Requires a multimodal model (image input support) for visual checks.",
+  "help_llm_binding_documentation": "Provider used for generating dataset and dashboard documentation.",
+  "help_llm_binding_git_commit": "Provider used for generating git commit messages.",
+  "help_llm_prompt_documentation": "System prompt template for documentation generation. Defines how the LLM should structure dataset descriptions.",
+  "help_llm_prompt_dashboard_validation": "System prompt template for dashboard validation. Instructs the LLM how to analyze dashboard screenshots and data.",
+  "help_llm_prompt_git_commit": "System prompt template for commit message generation. Guides the style and format of commit messages.",
+  "help_migration_cron": "Cron expression for automatic cross-environment ID synchronization. Format: minute hour day month weekday. Example: 0 2 * * * (daily at 2 AM UTC).",
+  "help_migration_sync_now": "Triggers an immediate synchronization of Dashboard, Chart, and Dataset IDs across all configured environments.",
+  "help_feature_dataset_review": "Enable the Dataset Review workflow: automatic review, clarification sessions, and SQL execution for imported datasets.",
+  "help_feature_health_monitor": "Enable dashboard health monitoring: validation reports, status aggregation, and health summary display."
 }