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/models/project.py

@@ -8,6 +8,7 @@ from fabledassistant.models.base import TimestampMixin, SoftDeleteMixin
 
 class ProjectStatus(str, enum.Enum):
     active = "active"
+    paused = "paused"
     completed = "completed"
     archived = "archived"