Add doc-service tests, AI category suggestions, LM Studio default

- pytest suite for doc-service: 20+ tests covering category CRUD,
  document upload/get/delete/patch, ownership isolation, category
  assignment, AI processing (mock), and live PDF tests (auto-skipped
  when tests/pdfs/ is empty)
- Minimal in-memory PDF builder in conftest so tests run without any
  fixture files; real PDFs can be dropped into tests/pdfs/ to activate
  live extraction tests
- AI prompt updated to return suggested_categories (2–5 short names)
- Frontend: SuggestionChip component in DocumentRow shows AI-suggested
  categories after processing; "Assign" links to an existing category,
  "Create & Assign" creates it first, ✕ dismisses locally
- Default AI provider changed to LM Studio at
  http://host.docker.internal:1234/v1 (host.docker.internal resolves
  to the macOS host from inside Docker Desktop)
- tests/pdfs/ directory tracked via .gitkeep; *.pdf excluded by .gitignore

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/core/app_config.py

@@ -31,13 +31,15 @@ class OllamaConfig(BaseModel):
 
 
 class LMStudioConfig(BaseModel):
-    base_url: str = "http://192.168.1.x:1234/v1"
+    # host.docker.internal resolves to the host from inside Docker (macOS/Windows).
+    # For local dev outside Docker, use http://localhost:1234/v1 instead.
+    base_url: str = "http://host.docker.internal:1234/v1"
     model: str = "local-model"
-    api_key: str = ""
+    api_key: str = "lm-studio"
 
 
 class AIConfig(BaseModel):
-    provider: str = "anthropic"
+    provider: str = "lmstudio"
     anthropic: AnthropicConfig = AnthropicConfig()
     ollama: OllamaConfig = OllamaConfig()
     lmstudio: LMStudioConfig = LMStudioConfig()