Add DeepSeek/xAI/OpenAI-compatible sentiment providers; custom dark dropdown

Providers (admin-switchable, no redeploy):
- DeepSeek and any OpenAI-compatible endpoint (OpenRouter, Together,
  Groq, local Ollama) via a generic Chat Completions adapter + base_url
- xAI Grok with Live Search (search_parameters web+X, citations) —
  grounded tier alongside OpenAI and Gemini
- DeepSeek / generic compatible endpoints are ungrounded (no web
  search); UI shows an amber warning and labels each provider's grounding
- Optional env fallbacks DEEPSEEK_API_KEY / XAI_API_KEY

UI: replace native <select> (unstyleable white popup on Windows) with a
custom dark Dropdown component everywhere — sentiment provider, scanner
filters, market sort, indicators, admin universe, user role.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

app/schemas/admin.py

@@ -70,9 +70,10 @@ class ActivationConfigUpdate(BaseModel):
 class SentimentConfigUpdate(BaseModel):
     """Runtime sentiment LLM config. api_key is write-only; omit/empty to keep
     the stored key."""
-    provider: Literal["openai", "gemini"] | None = None
+    provider: Literal["openai", "gemini", "deepseek", "xai", "openai_compatible"] | None = None
     model: str | None = Field(default=None, max_length=100)
     api_key: str | None = Field(default=None, max_length=400)
+    base_url: str | None = Field(default=None, max_length=300)
 
 
 class SentimentTestRequest(BaseModel):