feat(agent-chat): complete core workflow and strengthen auth rate limiting

backend/src/core/config/settings.py

@@ -132,6 +132,14 @@ class SupabaseSettings(BaseModel):
         return self.public_url
 
 
+class StorageSettings(BaseModel):
+    provider: Literal["supabase"] = "supabase"
+    bucket: str = Field(default="agent-chat-attachments", min_length=3, max_length=63)
+    signed_url_ttl_seconds: int = Field(default=600, ge=60, le=3600)
+    max_file_size_mb: int = Field(default=20, ge=1, le=200)
+    retention_days: int = Field(default=30, ge=1, le=3650)
+
+
 class DatabaseSettings(BaseModel):
     host: str = "localhost"
     port: int = 5432
@@ -163,6 +171,7 @@ class Settings(BaseSettings):
     cors: CorsSettings = CorsSettings()
     redis: RedisSettings = RedisSettings()
     supabase: SupabaseSettings = SupabaseSettings()
+    storage: StorageSettings = StorageSettings()
     celery: CelerySettings = CelerySettings()
     database: DatabaseSettings = DatabaseSettings()