Add AI chat with skills management and model provider settings

- Model providers for OpenAI and Anthropic message formats (e.g. via
  Sub2API), with API keys encrypted at rest and never returned
- Skills tree with markdown editing, priorities, drafts, publishing and
  rollback; only published skills enter the chat prompt
- Chat page with tool calls over collected news and Binance USD-M
  klines, a persisted CONTEXT.md, reset, and stop-generation support
- Chat may only create skill drafts and update CONTEXT.md; publishing,
  deleting, moving and priority changes stay with the user

backend/src/alphapulse/models/__init__.py

@@ -1,9 +1,14 @@
 from alphapulse.core.db import Base
+from alphapulse.models.ai import AiProvider, ChatMessage, ChatSession, ContextVersion, SkillNode, SkillVersion
 from alphapulse.models.market import Kline, KlineSyncState, MarketSymbol
 from alphapulse.models.news import CrawlLog, CrawlRun, NewsItem, NewsRevision, RawBody, RawResponse, Source
 
 __all__ = [
+    "AiProvider",
     "Base",
+    "ChatMessage",
+    "ChatSession",
+    "ContextVersion",
     "CrawlLog",
     "CrawlRun",
     "Kline",
@@ -13,5 +18,7 @@ __all__ = [
     "NewsRevision",
     "RawBody",
     "RawResponse",
+    "SkillNode",
+    "SkillVersion",
     "Source",
 ]