feat(message): add comprehensive message system with database migrations and health checks

- Add database migrations for message base table with indexes for efficient querying
- Implement rich content support with attachment, embed, and poll tables
- Create social features including reactions, bookmarks, mentions, and read states
- Add thread management with participant tracking and resolution capabilities
- Include article posts with title, cover image, tags, and engagement metrics
- Support scheduled messages, stickers, forwards, and interactive components
- Fix UUID defaults and ensure proper uniqueness constraints for drafts
- Add gRPC health server for imks and health check client for appks connectivity
- Replace non-connectable 0.0.0.0 addresses with localhost in service discovery
- Normalize addresses during RPC configuration to handle bind address issues

rpc/health.rs

@@ -0,0 +1,63 @@
+//! gRPC health module for imks.
+//!
+//! Provides:
+//! - imks's own gRPC health server (for external health checks)
+//! - Health check client for probing appks health (5s interval)
+
+use std::net::SocketAddr;
+use std::time::Duration;
+
+use tonic_health::pb::HealthCheckRequest;
+use tonic_health::pb::health_check_response::ServingStatus;
+use tonic_health::pb::health_client::HealthClient;
+
+use crate::{ImksError, ImksResult};
+
+/// Start imks's own gRPC health server on the given address.
+///
+/// Reports `SERVING` for the overall server status (empty service name).
+pub async fn start_health_server(addr: SocketAddr) -> ImksResult<()> {
+    let (reporter, health_service) = tonic_health::server::health_reporter();
+
+    // Empty service name = overall server health.
+    // reporter is an owned handle — the server will remain SERVING
+    // indefinitely unless a caller updates the status.
+    reporter
+        .set_service_status("", tonic_health::ServingStatus::Serving)
+        .await;
+
+    tracing::info!(%addr, "imks gRPC health server started");
+
+    tonic::transport::Server::builder()
+        .add_service(health_service)
+        .serve(addr)
+        .await
+        .map_err(|e| ImksError::Internal(format!("Health gRPC server: {e:?}")))?;
+
+    Ok(())
+}
+
+/// Check appks health by opening a short-lived gRPC connection and calling `Check`.
+///
+/// Uses a 3-second connect timeout to fail fast when appks is unreachable.
+pub async fn check_appks_health(addr: &str) -> ImksResult<bool> {
+    let endpoint = tonic::transport::Endpoint::from_shared(addr.to_string())
+        .map_err(|e| ImksError::Internal(format!("health endpoint: {e}")))?;
+
+    // Short-lived connection for health probe only
+    let channel = endpoint
+        .connect_timeout(Duration::from_secs(3))
+        .connect()
+        .await
+        .map_err(ImksError::GrpcTransport)?;
+
+    let mut client = HealthClient::new(channel);
+    let resp = client
+        .check(HealthCheckRequest {
+            service: "".to_string(),
+        })
+        .await
+        .map_err(ImksError::GrpcStatus)?;
+
+    Ok(resp.into_inner().status == ServingStatus::Serving as i32)
+}