feat(taskflow): add core task API, storage persistence, csv export, stats page, and test coverage

app/domain/__init__.py

@@ -0,0 +1,8 @@
+from app.domain.tasks import InvalidTransitionError, complete_task, start_task
+
+__all__ = [
+    "InvalidTransitionError",
+    "start_task",
+    "complete_task",
+]
+

app/domain/tasks.py

@@ -0,0 +1,60 @@
+from __future__ import annotations
+
+from datetime import datetime, UTC
+
+from app.storage.models import StoredTask
+
+
+class InvalidTransitionError(Exception):
+    """Raised when task workflow transition is invalid."""
+
+
+def start_task(task: StoredTask) -> StoredTask:
+    """
+    Transition task from backlog → in_progress.
+
+    Rules:
+    - Idempotent: if already started, do not change started_at
+    - Cannot start a task that is already done
+    """
+    # already done → invalid
+    if task.done_at is not None:
+        raise InvalidTransitionError("invalid_transaction")
+
+    # already started → idempotent (no change)
+    if task.started_at is not None:
+        return task
+
+    return StoredTask(
+        id=task.id,
+        title=task.title,
+        created_at=task.created_at,
+        started_at=datetime.now(UTC),
+        done_at=task.done_at,
+    )
+
+
+def complete_task(task: StoredTask) -> StoredTask:
+    """
+    Transition task from in_progress → done.
+
+    Rules:
+    - Allowed only if task is started
+    - Idempotent: if already done, do not change done_at
+    """
+    # not started → invalid
+    if task.started_at is None:
+        raise InvalidTransitionError("invalid_transaction")
+
+    # already done → idempotent
+    if task.done_at is not None:
+        return task
+
+    return StoredTask(
+        id=task.id,
+        title=task.title,
+        created_at=task.created_at,
+        started_at=task.started_at,
+        done_at=datetime.now(UTC),
+    )
+