fix(status-enum): add paused to ProjectStatus, validate, fix progress

Drift-audit Group 6 + Group 5 #6 (enum-extension / status drift):

- ProjectStatus gains 'paused' — routes and frontend already treated it as
  first-class, but the enum (the source of truth) omitted it and the error
  strings lied. A future CHECK derived from the enum would have rejected
  existing paused rows.
- create_project/update_project now validate status via ProjectStatus at the
  service layer (canonical gate; notes.status has no DB CHECK), so the MCP
  create/update_project path can't persist a typo'd status. MCP docstrings
  realigned to the 4-value domain; route error strings corrected.
- get_milestone_progress: cancelled tasks are excluded from the percent
  denominator (and now reported in status_counts), so a milestone whose only
  open task was cancelled reaches 100% instead of stalling below it.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/fabledassistant/routes/projects.py

@@ -53,7 +53,7 @@ async def create_project_route():
         return jsonify({"error": "title is required"}), 400
     status = data.get("status", "active")
     if status not in ("active", "paused", "completed", "archived"):
-        return jsonify({"error": "status must be 'active', 'completed', or 'archived'"}), 400
+        return jsonify({"error": "status must be 'active', 'paused', 'completed', or 'archived'"}), 400
     project = await create_project(
         uid,
         title=data["title"],
@@ -90,7 +90,7 @@ async def update_project_route(project_id: int):
     allowed = {"title", "description", "goal", "status", "color"}
     fields = {k: (v if v is not None else "") for k, v in data.items() if k in allowed}
     if "status" in fields and fields["status"] not in ("active", "paused", "completed", "archived"):
-        return jsonify({"error": "status must be 'active', 'completed', or 'archived'"}), 400
+        return jsonify({"error": "status must be 'active', 'paused', 'completed', or 'archived'"}), 400
     project = await update_project(uid, project_id, **fields)
     if project is None:
         return not_found("Project")