chore(infra): add gRPC layer, update protobufs, remove immediate module

- Add gRPC service modules: auth, channel, channel settings, member,
  permission
- Update protobuf definitions and generated code
- Remove immediate/ real-time module (superseded by IM service)
- Update etcd discovery and registration
- Update cache, error, config, and build infrastructure
- Add ADR documentation
- Update OpenAPI spec

error.rs

@@ -14,9 +14,6 @@ pub enum AppError {
     #[error("redis error: {0}")]
     Redis(#[from] redis::RedisError),
 
-    #[error("r2d2 error: {0}")]
-    R2d2(#[from] r2d2::Error),
-
     #[error("json error: {0}")]
     Json(#[from] serde_json::Error),
 
@@ -131,6 +128,7 @@ impl actix_web::ResponseError for AppError {
             | AppError::InvalidEmailCode
             | AppError::RsaDecodeError
             | AppError::RsaGenerationError => StatusCode::BAD_REQUEST,
+            AppError::Database(e) => db_error_status_code(e),
             AppError::PasswordHashError(_) => StatusCode::INTERNAL_SERVER_ERROR,
             _ => StatusCode::INTERNAL_SERVER_ERROR,
         }
@@ -139,6 +137,7 @@ impl actix_web::ResponseError for AppError {
     fn error_response(&self) -> HttpResponse {
         let status = self.status_code();
         let message = if status == actix_web::http::StatusCode::INTERNAL_SERVER_ERROR {
+            tracing::error!(?self, "internal server error");
             "internal server error".to_string()
         } else {
             self.to_string()
@@ -146,3 +145,25 @@ impl actix_web::ResponseError for AppError {
         HttpResponse::build(status).json(serde_json::json!({ "error": message }))
     }
 }
+
+fn db_error_status_code(e: &sqlx::Error) -> actix_web::http::StatusCode {
+    use actix_web::http::StatusCode;
+    match e {
+        sqlx::Error::Database(db_err) => {
+            match db_err.code().as_ref().map(|c| c.as_ref()) {
+                // unique_violation
+                Some("23505") => StatusCode::CONFLICT,
+                // foreign_key_violation
+                Some("23503") => StatusCode::CONFLICT,
+                // check_violation
+                Some("23514") => StatusCode::BAD_REQUEST,
+                // not_null_violation
+                Some("23502") => StatusCode::BAD_REQUEST,
+                // exclusion_violation
+                Some("23P01") => StatusCode::CONFLICT,
+                _ => StatusCode::INTERNAL_SERVER_ERROR,
+            }
+        }
+        _ => StatusCode::INTERNAL_SERVER_ERROR,
+    }
+}