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/models/propose.py

@@ -0,0 +1,29 @@
+from sqlalchemy import Column, Integer, String, Text, DateTime, ForeignKey, Enum
+from sqlalchemy.sql import func
+from app.core.config import Base
+import enum
+
+
+class ProposeStatus(str, enum.Enum):
+    OPEN = "open"
+    ACCEPTED = "accepted"
+    REJECTED = "rejected"
+
+
+class Propose(Base):
+    __tablename__ = "proposes"
+
+    id = Column(Integer, primary_key=True, index=True)
+    propose_code = Column(String(64), nullable=True, unique=True, index=True)
+    title = Column(String(255), nullable=False)
+    description = Column(Text, nullable=True)
+    status = Column(Enum(ProposeStatus), default=ProposeStatus.OPEN)
+
+    project_id = Column(Integer, ForeignKey("projects.id"), nullable=False)
+    created_by_id = Column(Integer, ForeignKey("users.id"), nullable=True)
+
+    # Populated server-side after accept; links to the generated feature story task
+    feat_task_id = Column(String(64), nullable=True)
+
+    created_at = Column(DateTime(timezone=True), server_default=func.now())
+    updated_at = Column(DateTime(timezone=True), onupdate=func.now())