feat(05-01): create discussion service with CRUD operations

Add discussion.py service with create_discussion, get_discussion,
get_active_discussion, list_discussions, and complete_discussion.
Uses selectinload for eager loading of rounds, messages, and consensus.

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

src/moai/core/services/__init__.py

@@ -4,6 +4,22 @@ Services encapsulate database operations and business rules,
 providing a clean interface for handlers to use.
 """
 
+from moai.core.services.discussion import (
+    complete_discussion,
+    create_discussion,
+    get_active_discussion,
+    get_discussion,
+    list_discussions,
+)
 from moai.core.services.project import create_project, get_project, list_projects
 
-__all__ = ["create_project", "get_project", "list_projects"]
+__all__ = [
+    "complete_discussion",
+    "create_discussion",
+    "create_project",
+    "get_active_discussion",
+    "get_discussion",
+    "get_project",
+    "list_discussions",
+    "list_projects",
+]

src/moai/core/services/discussion.py

@@ -0,0 +1,124 @@
+"""Discussion service for MoAI.
+
+Provides CRUD operations for discussions, rounds, and messages.
+"""
+
+from sqlalchemy import select
+from sqlalchemy.orm import selectinload
+
+from moai.core.database import get_session
+from moai.core.models import (
+    Discussion,
+    DiscussionStatus,
+    DiscussionType,
+    Round,
+)
+
+
+async def create_discussion(
+    project_id: str,
+    question: str,
+    discussion_type: DiscussionType,
+) -> Discussion:
+    """Create a new discussion within a project.
+
+    Args:
+        project_id: The parent project's UUID.
+        question: The question or topic being discussed.
+        discussion_type: Whether this is OPEN or DISCUSS mode.
+
+    Returns:
+        The created Discussion object.
+    """
+    async with get_session() as session:
+        discussion = Discussion(
+            project_id=project_id,
+            question=question,
+            type=discussion_type,
+        )
+        session.add(discussion)
+        await session.flush()
+        await session.refresh(discussion)
+        return discussion
+
+
+async def get_discussion(discussion_id: str) -> Discussion | None:
+    """Get a discussion by ID with eager loading of rounds and messages.
+
+    Args:
+        discussion_id: The discussion's UUID.
+
+    Returns:
+        The Discussion object if found, None otherwise.
+    """
+    async with get_session() as session:
+        result = await session.execute(
+            select(Discussion)
+            .where(Discussion.id == discussion_id)
+            .options(
+                selectinload(Discussion.rounds).selectinload(Round.messages),
+                selectinload(Discussion.consensus),
+            )
+        )
+        return result.scalar_one_or_none()
+
+
+async def get_active_discussion(project_id: str) -> Discussion | None:
+    """Get the active discussion for a project.
+
+    Args:
+        project_id: The project's UUID.
+
+    Returns:
+        The active Discussion object if found, None otherwise.
+    """
+    async with get_session() as session:
+        result = await session.execute(
+            select(Discussion)
+            .where(Discussion.project_id == project_id)
+            .where(Discussion.status == DiscussionStatus.ACTIVE)
+            .options(
+                selectinload(Discussion.rounds).selectinload(Round.messages),
+            )
+        )
+        return result.scalar_one_or_none()
+
+
+async def list_discussions(project_id: str) -> list[Discussion]:
+    """List all discussions for a project ordered by creation date (newest first).
+
+    Args:
+        project_id: The project's UUID.
+
+    Returns:
+        List of Discussion objects.
+    """
+    async with get_session() as session:
+        result = await session.execute(
+            select(Discussion)
+            .where(Discussion.project_id == project_id)
+            .order_by(Discussion.created_at.desc())
+        )
+        return list(result.scalars().all())
+
+
+async def complete_discussion(discussion_id: str) -> Discussion | None:
+    """Mark a discussion as completed.
+
+    Args:
+        discussion_id: The discussion's UUID.
+
+    Returns:
+        The updated Discussion object if found, None otherwise.
+    """
+    async with get_session() as session:
+        result = await session.execute(select(Discussion).where(Discussion.id == discussion_id))
+        discussion = result.scalar_one_or_none()
+
+        if discussion is None:
+            return None
+
+        discussion.status = DiscussionStatus.COMPLETED
+        await session.flush()
+        await session.refresh(discussion)
+        return discussion