feat(agent): add LLM-powered metadata enrichment system with AgentCLI and PostProcessor

Introduce an agent skill framework for LLM-driven metadata enrichment:

- AgentCLI (py/agent_cli/): in-process wrappers around internal services
  using standard relative imports, eliminating the need for sys.path hacks
- LLMService: centralized BYOK (bring-your-own-key) LLM client supporting
  OpenAI, Ollama, and custom OpenAI-compatible endpoints
- PostProcessor: deterministic engine that applies LLM output via AgentCLI
  (replaces old handler.py + _BASE_MODEL_ALIASES approach)
- SkillRegistry: filesystem-based skill discovery (skill.yaml + prompt.md)
- AgentService: orchestrates skill execution with WebSocket progress
- Frontend AgentManager: WebSocket listeners, skill execution, config UI
- Context menu entries (single + bulk) for "Enrich Metadata (Agent)"
- Settings UI for AI Provider configuration (BYOK)
- Full i18n support across 9 locales

Bug fixes found during review:
- aiohttp.web.json_response: status_code= -> status=
- settings_modal cancelEditApiKey: wrong argument position
- AgentManager.isLlmConfigured: allow Ollama without API key
- PostProcessor._merge_tags: lowercase all tags to match TagUpdateService

py/services/agent/skills/enrich_hf_metadata/skill.yaml

@@ -0,0 +1,47 @@
+name: enrich_hf_metadata
+title: "Enrich Metadata from HuggingFace"
+description: >
+  Parse the HuggingFace model card via LLM to extract description, trigger
+  words, base model, tags, and preview image URL.  Updates .metadata.json
+  and downloads the preview thumbnail.
+llm_required: true
+model_type_filter: ["lora", "checkpoint", "embedding"]
+input_schema:
+  type: object
+  properties:
+    model_paths:
+      type: array
+      items:
+        type: string
+  required:
+    - model_paths
+output_schema:
+  type: object
+  properties:
+    model_path:
+      type: string
+    base_model:
+      type: string
+    trigger_words:
+      type: array
+      items:
+        type: string
+    description:
+      type: string
+    tags:
+      type: array
+      items:
+        type: string
+    preview_url:
+      type: string
+    confidence:
+      type: string
+      enum: ["high", "medium", "low"]
+  required:
+    - model_path
+    - confidence
+permissions:
+  write_metadata: true
+  write_previews: true
+  network_domains:
+    - "huggingface.co"