feat: add conversation persistence and context management layer

- SQLAlchemy async database layer (SQLite dev, PostgreSQL prod)
- Conversation and Message models with UUID primary keys
- Token counting utilities using litellm
- Context summarization at 80% token threshold
- REST API endpoints for multi-turn conversations
- 19 conversation tests, 6 token tests (176 total passing)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

webber-api/src/domains/conversations/models.py

@@ -0,0 +1,72 @@
+"""
+Database models for conversations.
+
+Following core-api patterns: SQLAlchemy 2.0 with async support.
+"""
+from datetime import datetime
+from uuid import UUID, uuid4
+
+from sqlalchemy import ForeignKey, String, Text
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from src.db.models import Base
+
+
+class Conversation(Base):
+    """
+    A conversation session with an agent.
+
+    Tracks message history, token usage, and metadata.
+    """
+    __tablename__ = "conversations"
+
+    id: Mapped[UUID] = mapped_column(primary_key=True, default=uuid4)
+    user_id: Mapped[str] = mapped_column(String(255), index=True)
+    agent_type: Mapped[str] = mapped_column(String(50), default="explore", insert_default="explore")
+    title: Mapped[str | None] = mapped_column(String(255), nullable=True, default=None)
+    working_dir: Mapped[str] = mapped_column(String(1024), default=".", insert_default=".")
+    total_tokens: Mapped[int] = mapped_column(default=0, insert_default=0)
+    created_at: Mapped[datetime] = mapped_column(default=datetime.utcnow)
+    updated_at: Mapped[datetime | None] = mapped_column(
+        default=datetime.utcnow,
+        onupdate=datetime.utcnow,
+        nullable=True
+    )
+
+    # Relationships
+    messages: Mapped[list["Message"]] = relationship(
+        back_populates="conversation",
+        cascade="all, delete-orphan",
+        order_by="Message.created_at",
+    )
+
+    def __repr__(self) -> str:
+        return f"<Conversation {self.id} agent={self.agent_type}>"
+
+
+class Message(Base):
+    """
+    A single message in a conversation.
+
+    Tracks role, content, token count, and summarization state.
+    """
+    __tablename__ = "messages"
+
+    id: Mapped[UUID] = mapped_column(primary_key=True, default=uuid4)
+    conversation_id: Mapped[UUID] = mapped_column(
+        ForeignKey("conversations.id", ondelete="CASCADE"),
+        index=True
+    )
+    role: Mapped[str] = mapped_column(String(20))  # user, assistant, system, summary
+    content: Mapped[str] = mapped_column(Text)
+    token_count: Mapped[int] = mapped_column(default=0, insert_default=0)
+    is_summary: Mapped[bool] = mapped_column(default=False, insert_default=False)
+    summarizes_up_to: Mapped[UUID | None] = mapped_column(nullable=True, default=None)
+    created_at: Mapped[datetime] = mapped_column(default=datetime.utcnow)
+
+    # Relationships
+    conversation: Mapped["Conversation"] = relationship(back_populates="messages")
+
+    def __repr__(self) -> str:
+        preview = self.content[:30] + "..." if len(self.content) > 30 else self.content
+        return f"<Message {self.role}: {preview}>"