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/router.py

@@ -0,0 +1,189 @@
+"""
+REST API routes for conversations.
+"""
+from uuid import UUID
+
+from fastapi import APIRouter, Depends, HTTPException
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from src.db import get_session
+from src.domains.conversations.schemas import (
+    AddMessageRequest,
+    AddMessageResponse,
+    ConversationDetailResponse,
+    ConversationListResponse,
+    ConversationResponse,
+    CreateConversationRequest,
+    MessageResponse,
+)
+from src.domains.conversations.service import ConversationService
+from src.shared.auth import require_auth
+from src.shared.logging import logged, get_logger
+
+logger = get_logger(__name__)
+
+router = APIRouter(prefix="/conversations", tags=["Conversations"])
+
+
+@router.post("/", response_model=ConversationResponse, status_code=201)
+@logged()
+async def create_conversation(
+    request: CreateConversationRequest,
+    session: AsyncSession = Depends(get_session),
+    user=Depends(require_auth),
+) -> ConversationResponse:
+    """
+    Create a new conversation.
+
+    Starts an empty conversation with the specified agent type.
+    """
+    service = ConversationService(session)
+    conversation = await service.create(
+        user_id=user.id,
+        agent_type=request.agent_type,
+        working_dir=request.working_dir,
+        title=request.title,
+    )
+    return ConversationResponse.model_validate(conversation)
+
+
+@router.get("/", response_model=ConversationListResponse)
+@logged()
+async def list_conversations(
+    limit: int = 50,
+    offset: int = 0,
+    session: AsyncSession = Depends(get_session),
+    user=Depends(require_auth),
+) -> ConversationListResponse:
+    """
+    List user's conversations.
+
+    Returns conversations sorted by most recently updated.
+    """
+    service = ConversationService(session)
+    conversations, total = await service.list_by_user(
+        user_id=user.id,
+        limit=limit,
+        offset=offset,
+    )
+    return ConversationListResponse(
+        conversations=[ConversationResponse.model_validate(c) for c in conversations],
+        total=total,
+    )
+
+
+@router.get("/{conversation_id}", response_model=ConversationDetailResponse)
+@logged()
+async def get_conversation(
+    conversation_id: UUID,
+    session: AsyncSession = Depends(get_session),
+    user=Depends(require_auth),
+) -> ConversationDetailResponse:
+    """
+    Get conversation with all messages.
+
+    Returns conversation metadata and full message history.
+    """
+    service = ConversationService(session)
+    conversation = await service.get_with_messages(conversation_id)
+
+    if not conversation:
+        raise HTTPException(status_code=404, detail="Conversation not found")
+
+    if conversation.user_id != user.id:
+        raise HTTPException(status_code=403, detail="Not authorized")
+
+    return ConversationDetailResponse.model_validate(conversation)
+
+
+@router.delete("/{conversation_id}", status_code=204)
+@logged()
+async def delete_conversation(
+    conversation_id: UUID,
+    session: AsyncSession = Depends(get_session),
+    user=Depends(require_auth),
+) -> None:
+    """
+    Delete a conversation and all its messages.
+    """
+    service = ConversationService(session)
+    conversation = await service.get(conversation_id)
+
+    if not conversation:
+        raise HTTPException(status_code=404, detail="Conversation not found")
+
+    if conversation.user_id != user.id:
+        raise HTTPException(status_code=403, detail="Not authorized")
+
+    await service.delete(conversation_id)
+
+
+@router.post("/{conversation_id}/messages", response_model=AddMessageResponse)
+@logged()
+async def add_message(
+    conversation_id: UUID,
+    request: AddMessageRequest,
+    session: AsyncSession = Depends(get_session),
+    user=Depends(require_auth),
+) -> AddMessageResponse:
+    """
+    Add a message to a conversation and get agent response.
+
+    This is the main endpoint for continuing conversations.
+    It:
+    1. Adds the user message
+    2. Checks if summarization is needed
+    3. Builds context from conversation history
+    4. Gets agent response
+    5. Adds agent response to conversation
+    6. Returns both messages
+    """
+    service = ConversationService(session)
+
+    # Verify conversation exists and user owns it
+    conversation = await service.get(conversation_id)
+    if not conversation:
+        raise HTTPException(status_code=404, detail="Conversation not found")
+
+    if conversation.user_id != user.id:
+        raise HTTPException(status_code=403, detail="Not authorized")
+
+    # Add user message
+    user_message = await service.add_message(
+        conversation_id=conversation_id,
+        role="user",
+        content=request.content,
+    )
+
+    # Check if summarization needed before getting response
+    summarized = await service.summarize_if_needed(conversation_id)
+
+    # Get agent response with context
+    try:
+        response_text = await service.get_agent_response(
+            conversation_id=conversation_id,
+            user_message=request.content,
+        )
+    except Exception as e:
+        logger.exception(f"Agent response failed: {e}")
+        raise HTTPException(
+            status_code=500,
+            detail=f"Agent error: {str(e)}"
+        )
+
+    # Add assistant message
+    assistant_message = await service.add_message(
+        conversation_id=conversation_id,
+        role="assistant",
+        content=response_text,
+    )
+
+    # Get updated conversation for total tokens
+    conversation = await service.get(conversation_id)
+
+    return AddMessageResponse(
+        user_message=MessageResponse.model_validate(user_message),
+        assistant_message=MessageResponse.model_validate(assistant_message),
+        total_tokens=conversation.total_tokens if conversation else 0,
+        summarized=summarized,
+    )