feat: add code-based meetings router with participant/attend support

- New dedicated meetings.py router with full CRUD (list/get/create/update/delete)
- All endpoints accept meeting_code or numeric id
- MeetingParticipant model for tracking meeting attendance
- POST /meetings/{id}/attend adds current user to participant list
- Serialization includes participants list, project_code, milestone_code
- Creator auto-added as participant on meeting creation
- Registered in main.py alongside existing routers

app/models/meeting.py

@@ -1,4 +1,4 @@
-from sqlalchemy import Column, Integer, String, Text, DateTime, ForeignKey, Enum
+from sqlalchemy import Column, Integer, String, Text, DateTime, ForeignKey, Enum, UniqueConstraint
 from sqlalchemy.orm import relationship
 from sqlalchemy.sql import func
 from app.core.config import Base
@@ -35,3 +35,19 @@ class Meeting(Base):
     
     created_at = Column(DateTime(timezone=True), server_default=func.now())
     updated_at = Column(DateTime(timezone=True), onupdate=func.now())
+
+    participants = relationship("MeetingParticipant", back_populates="meeting", cascade="all, delete-orphan")
+
+
+class MeetingParticipant(Base):
+    __tablename__ = "meeting_participants"
+    __table_args__ = (
+        UniqueConstraint("meeting_id", "user_id", name="uq_meeting_participant"),
+    )
+
+    id = Column(Integer, primary_key=True, index=True)
+    meeting_id = Column(Integer, ForeignKey("meetings.id", ondelete="CASCADE"), nullable=False)
+    user_id = Column(Integer, ForeignKey("users.id"), nullable=False)
+    joined_at = Column(DateTime(timezone=True), server_default=func.now())
+
+    meeting = relationship("Meeting", back_populates="participants")