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/schemas.py

@@ -0,0 +1,79 @@
+"""
+Pydantic schemas for conversation API.
+"""
+from datetime import datetime
+from uuid import UUID
+
+from pydantic import BaseModel, Field
+
+
+# === Request Schemas ===
+
+class CreateConversationRequest(BaseModel):
+    """Request to create a new conversation."""
+    agent_type: str = Field(default="explore", description="Agent type to use")
+    working_dir: str = Field(default=".", description="Working directory for agent")
+    title: str | None = Field(default=None, description="Optional conversation title")
+
+
+class AddMessageRequest(BaseModel):
+    """Request to add a message to a conversation."""
+    content: str = Field(..., min_length=1, description="Message content")
+
+
+# === Response Schemas ===
+
+class MessageResponse(BaseModel):
+    """Response for a single message."""
+    id: UUID
+    role: str
+    content: str
+    token_count: int
+    is_summary: bool
+    created_at: datetime
+
+    model_config = {"from_attributes": True}
+
+
+class ConversationResponse(BaseModel):
+    """Response for conversation metadata."""
+    id: UUID
+    agent_type: str
+    title: str | None
+    working_dir: str
+    total_tokens: int
+    created_at: datetime
+    updated_at: datetime | None
+
+    model_config = {"from_attributes": True}
+
+
+class ConversationDetailResponse(BaseModel):
+    """Response for conversation with messages."""
+    id: UUID
+    agent_type: str
+    title: str | None
+    working_dir: str
+    total_tokens: int
+    created_at: datetime
+    updated_at: datetime | None
+    messages: list[MessageResponse]
+
+    model_config = {"from_attributes": True}
+
+
+class ConversationListResponse(BaseModel):
+    """Response for listing conversations."""
+    conversations: list[ConversationResponse]
+    total: int
+
+
+class AddMessageResponse(BaseModel):
+    """Response after adding a message (includes agent response)."""
+    user_message: MessageResponse
+    assistant_message: MessageResponse
+    total_tokens: int
+    summarized: bool = Field(
+        default=False,
+        description="Whether context was summarized due to token limit"
+    )