Fix database migrations for Railway deployment

- Add InviteCode model import to alembic env.py
- Derive DATABASE_URL_SYNC from DATABASE_URL as a property
  so it uses the same Railway-provided connection string

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/alembic/env.py

@@ -9,7 +9,7 @@ from alembic import context
 
 # Import your models
 from app.core.database import Base
-from app.models import User, Team, Tree, Session, Attachment
+from app.models import User, Team, Tree, Session, Attachment, InviteCode
 from app.core.config import settings
 
 # this is the Alembic Config object

backend/app/core/config.py

@@ -11,7 +11,6 @@ class Settings(BaseSettings):
 
     # Database - Railway provides DATABASE_URL, we convert it for asyncpg
     DATABASE_URL: str = "postgresql+asyncpg://postgres:postgres@localhost:5432/patherly"
-    DATABASE_URL_SYNC: str = "postgresql://postgres:postgres@localhost:5432/patherly"
 
     @field_validator("DATABASE_URL", mode="before")
     @classmethod
@@ -21,13 +20,10 @@ class Settings(BaseSettings):
             return v.replace("postgresql://", "postgresql+asyncpg://", 1)
         return v
 
-    @field_validator("DATABASE_URL_SYNC", mode="before")
-    @classmethod
-    def ensure_sync_url(cls, v: str) -> str:
-        """Ensure sync URL uses standard postgresql prefix."""
-        if v.startswith("postgresql+asyncpg://"):
-            return v.replace("postgresql+asyncpg://", "postgresql://", 1)
-        return v
+    @property
+    def DATABASE_URL_SYNC(self) -> str:
+        """Get sync URL by removing asyncpg prefix from DATABASE_URL."""
+        return self.DATABASE_URL.replace("postgresql+asyncpg://", "postgresql://", 1)
 
     # JWT Settings
     SECRET_KEY: str = "your-secret-key-change-in-production-use-openssl-rand-hex-32"