feat: add Propose model/schema + DB enum migration scripts

- New Propose model (app/models/propose.py) with status enum (open/accepted/rejected)
- New Propose schemas (ProposeCreate/Update/Response) in schemas.py
- MySQL enum migration in main.py for milestone/task status columns
  - milestone: pending→open, deferred→closed, progressing→undergoing
  - task: progressing→undergoing
- Import propose model in startup for create_all
- Add started_at column migration for milestones

app/main.py

@@ -168,6 +168,45 @@ def _migrate_schema():
         if _has_table(db, "issues"):
             db.execute(text("DROP TABLE issues"))
 
+        # --- Milestone status enum migration (old -> new) ---
+        if _has_table(db, "milestones"):
+            # Alter enum column to accept new values
+            db.execute(text(
+                "ALTER TABLE milestones MODIFY COLUMN status "
+                "ENUM('open','pending','deferred','progressing','freeze','undergoing','completed','closed') "
+                "DEFAULT 'open'"
+            ))
+            # Migrate old values
+            db.execute(text("UPDATE milestones SET status='open' WHERE status='pending'"))
+            db.execute(text("UPDATE milestones SET status='closed' WHERE status='deferred'"))
+            db.execute(text("UPDATE milestones SET status='undergoing' WHERE status='progressing'"))
+            # Shrink enum to new-only values
+            db.execute(text(
+                "ALTER TABLE milestones MODIFY COLUMN status "
+                "ENUM('open','freeze','undergoing','completed','closed') "
+                "DEFAULT 'open'"
+            ))
+            # Add started_at if missing
+            if not _has_column(db, "milestones", "started_at"):
+                db.execute(text("ALTER TABLE milestones ADD COLUMN started_at DATETIME NULL"))
+
+        # --- Task status enum migration (old -> new) ---
+        if _has_table(db, "tasks"):
+            # Widen enum first
+            db.execute(text(
+                "ALTER TABLE tasks MODIFY COLUMN status "
+                "ENUM('open','pending','progressing','undergoing','completed','closed') "
+                "DEFAULT 'open'"
+            ))
+            # Migrate old values
+            db.execute(text("UPDATE tasks SET status='undergoing' WHERE status='progressing'"))
+            # Shrink enum to new-only values
+            db.execute(text(
+                "ALTER TABLE tasks MODIFY COLUMN status "
+                "ENUM('open','pending','undergoing','completed','closed') "
+                "DEFAULT 'open'"
+            ))
+
         db.commit()
     except Exception as e:
         db.rollback()
@@ -179,7 +218,7 @@ def _migrate_schema():
 @app.on_event("startup")
 def startup():
     from app.core.config import Base, engine, SessionLocal
-    from app.models import models, webhook, apikey, activity, milestone, notification, worklog, monitor, role_permission, task, support, meeting
+    from app.models import models, webhook, apikey, activity, milestone, notification, worklog, monitor, role_permission, task, support, meeting, propose
     Base.metadata.create_all(bind=engine)
     _migrate_schema()