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/ai.py

@@ -0,0 +1,102 @@
+from datetime import datetime
+
+from sqlalchemy import BigInteger, Boolean, DateTime, Float, ForeignKey, Integer, LargeBinary, String, Text, UniqueConstraint, func
+from sqlalchemy.dialects.postgresql import JSONB
+from sqlalchemy.orm import Mapped, mapped_column
+
+from alphapulse.core.db import Base
+
+
+class AiProvider(Base):
+    """模型接入配置；api_key 只以密文存储"""
+
+    __tablename__ = "ai_providers"
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    name: Mapped[str] = mapped_column(String(64), unique=True)
+    # openai：/v1/chat/completions；anthropic：/v1/messages
+    protocol: Mapped[str] = mapped_column(String(16))
+    base_url: Mapped[str] = mapped_column(String(255))
+    api_key_encrypted: Mapped[bytes] = mapped_column(LargeBinary)
+    api_key_hint: Mapped[str] = mapped_column(String(8))
+    model: Mapped[str] = mapped_column(String(128))
+    max_tokens: Mapped[int] = mapped_column(Integer, default=8192)
+    # 上下文上限（token），超过时拒绝发送而不是静默截断
+    context_window: Mapped[int] = mapped_column(Integer, default=200000)
+    temperature: Mapped[float | None] = mapped_column(Float)
+    is_default: Mapped[bool] = mapped_column(Boolean, default=False)
+    created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), server_default=func.now())
+    updated_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), server_default=func.now(), onupdate=func.now())
+
+
+class SkillNode(Base):
+    """skills 树：文件夹或 skill；内容在 skill_versions，生效的是 published_version_id 指向的版本"""
+
+    __tablename__ = "skill_nodes"
+    __table_args__ = (UniqueConstraint("parent_id", "name", postgresql_nulls_not_distinct=True),)
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    parent_id: Mapped[int | None] = mapped_column(ForeignKey("skill_nodes.id"))
+    # folder / skill
+    kind: Mapped[str] = mapped_column(String(8))
+    name: Mapped[str] = mapped_column(String(128))
+    sort_order: Mapped[int] = mapped_column(Integer, default=0)
+    priority: Mapped[int] = mapped_column(Integer, default=0)
+    enabled: Mapped[bool] = mapped_column(Boolean, default=True)
+    published_version_id: Mapped[int | None] = mapped_column(ForeignKey("skill_versions.id", use_alter=True))
+    created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), server_default=func.now())
+    updated_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), server_default=func.now(), onupdate=func.now())
+
+
+class SkillVersion(Base):
+    """只追加不修改；回退即重新发布旧版本"""
+
+    __tablename__ = "skill_versions"
+    __table_args__ = (UniqueConstraint("skill_id", "version_no"),)
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    skill_id: Mapped[int] = mapped_column(ForeignKey("skill_nodes.id", ondelete="CASCADE"))
+    version_no: Mapped[int] = mapped_column(Integer)
+    content: Mapped[str] = mapped_column(Text)
+    # user / chat
+    source: Mapped[str] = mapped_column(String(8))
+    note: Mapped[str] = mapped_column(Text, default="")
+    created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), server_default=func.now())
+
+
+class ContextVersion(Base):
+    """Chat 的 CONTEXT.md；最新一行即当前内容"""
+
+    __tablename__ = "context_versions"
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    content: Mapped[str] = mapped_column(Text)
+    # user / chat / reset
+    source: Mapped[str] = mapped_column(String(8))
+    created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), server_default=func.now())
+
+
+class ChatSession(Base):
+    __tablename__ = "chat_sessions"
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), server_default=func.now())
+    # Reset 时结束；未结束的只有一个，即当前会话
+    ended_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))
+
+
+class ChatMessage(Base):
+    """content 为与协议无关的格式：
+    user: {"text"}；assistant: {"text", "tool_calls": [{"id","name","arguments"}]}；tool: {"tool_call_id","name","result"}
+    """
+
+    __tablename__ = "chat_messages"
+
+    id: Mapped[int] = mapped_column(BigInteger, primary_key=True)
+    session_id: Mapped[int] = mapped_column(ForeignKey("chat_sessions.id"), index=True)
+    # user / assistant / tool
+    role: Mapped[str] = mapped_column(String(16))
+    content: Mapped[dict] = mapped_column(JSONB)
+    provider_id: Mapped[int | None] = mapped_column(ForeignKey("ai_providers.id", ondelete="SET NULL"))
+    usage: Mapped[dict | None] = mapped_column(JSONB)
+    created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), server_default=func.now())