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/config.py

@@ -24,6 +24,10 @@ class Settings(BaseSettings):
     openai_model: str = "gpt-4o-mini"
     openai_sentiment_batch_size: int = 5
 
+    # Sentiment Provider — DeepSeek / xAI (OpenAI-compatible; optional env fallback)
+    deepseek_api_key: str = ""
+    xai_api_key: str = ""
+
     # Fundamentals Provider — Financial Modeling Prep
     fmp_api_key: str = ""