perf(prop-grid-rs): claim N forecast hours concurrently

Replaces the serial claim→process loop with a JoinSet of N worker tasks,
each running the same claim→fetch→decode→score→complete flow against
grid_tasks. FOR UPDATE SKIP LOCKED already prevents contention between
workers so no additional coordination is needed.

talos5 is 4c/4t (i5-7260U); PROP_GRID_RS_PARALLELISM=4 turns the full
f01..f18 chain from ~18 min serial into roughly ~5 min. Per-task peak
memory after the streamed-bands refactor is ~250 Mi, so 4 concurrent is
~1 Gi — well under the 2 Gi container limit.

Postgres pool sized to parallelism + 2 so NOTIFY and an opportunistic
claim retry share the pool without blocking a worker transaction.

k8s/deployment-grid-rs.yaml

@@ -61,8 +61,17 @@ spec:
               value: "/data/scores"
             - name: RUST_LOG
               value: "info"
-            - name: PROP_GRID_RS_PG_CONNS
+            # talos5 is 4c/4t (i5-7260U). Claiming 4 forecast hours in
+            # parallel drops full-chain wall time from ~18 min serial to
+            # ~5 min. Each concurrent task peaks around 250 Mi after the
+            # streamed-bands refactor; 4×250 ≈ 1 Gi is comfortably under
+            # the 2 Gi memory limit.
+            - name: PROP_GRID_RS_PARALLELISM
               value: "4"
+            # Pool size = parallelism + 2 (NOTIFY + retry slack). Kept in
+            # sync with PROP_GRID_RS_PARALLELISM when either changes.
+            - name: PROP_GRID_RS_PG_CONNS
+              value: "6"
           envFrom:
             - secretRef:
                 name: prop-secrets

rust/prop_grid_rs/src/bin/worker.rs

@@ -2,11 +2,13 @@
 //!
 //! Flow:
 //! 1. Connect to Postgres (DATABASE_URL env).
-//! 2. Loop: claim one `grid_tasks` row (fh>0, status='queued'),
-//!    run the fetch → decode → score → write-scores-file chain, mark done +
-//!    `NOTIFY propagation_ready '<valid_time iso>'`. On error mark
+//! 2. Spawn N concurrent worker tasks (`PROP_GRID_RS_PARALLELISM`, default
+//!    = num_cpus). Each worker independently claims one `grid_tasks` row
+//!    (fh>0, status='queued') via `FOR UPDATE SKIP LOCKED`, runs the
+//!    fetch → decode → score → write-scores-file chain, and marks done +
+//!    `NOTIFY propagation_ready '<valid_time iso>'`. On error marks
 //!    failed; hourly cron reseeds.
-//! 3. Graceful shutdown on SIGTERM: requeue current row and exit.
+//! 3. Graceful shutdown on SIGTERM: workers exit after their current task.
 
 use std::path::PathBuf;
 use std::sync::Arc;
@@ -14,6 +16,7 @@ use std::sync::atomic::{AtomicBool, Ordering};
 use std::time::Duration;
 
 use prop_grid_rs::{db, fetcher::HrrrClient, pipeline};
+use tokio::task::JoinSet;
 use tracing::{error, info, warn};
 use tracing_subscriber::EnvFilter;
 
@@ -33,10 +36,18 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
         .unwrap_or_else(|_| PathBuf::from("/data/scores"));
     let hrrr_base = std::env::var("HRRR_BASE_URL")
         .unwrap_or_else(|_| prop_grid_rs::fetcher::HRRR_BASE_DEFAULT.to_string());
+    let parallelism: usize = std::env::var("PROP_GRID_RS_PARALLELISM")
+        .ok()
+        .and_then(|v| v.parse().ok())
+        .unwrap_or_else(num_cpus::get)
+        .max(1);
+    // +2 slots over parallelism: each worker holds one connection during
+    // claim/complete transactions; the spare lets NOTIFY and an opportunistic
+    // retry share the pool without blocking.
     let max_conn: u32 = std::env::var("PROP_GRID_RS_PG_CONNS")
         .ok()
         .and_then(|v| v.parse().ok())
-        .unwrap_or(4);
+        .unwrap_or((parallelism as u32) + 2);
 
     let pool = db::connect(&database_url, max_conn).await?;
     let client = Arc::new(HrrrClient::new(hrrr_base)?);
@@ -44,8 +55,37 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
     let shutdown = Arc::new(AtomicBool::new(false));
     spawn_signal_handler(shutdown.clone());
 
-    info!(scores_dir = %scores_dir.display(), "prop-grid-rs started");
+    info!(
+        scores_dir = %scores_dir.display(),
+        parallelism,
+        pg_conns = max_conn,
+        "prop-grid-rs started"
+    );
 
+    let mut set: JoinSet<()> = JoinSet::new();
+    for worker_id in 0..parallelism {
+        let pool = pool.clone();
+        let client = client.clone();
+        let scores_dir = scores_dir.clone();
+        let shutdown = shutdown.clone();
+        set.spawn(async move {
+            worker_loop(worker_id, pool, client, scores_dir, shutdown).await;
+        });
+    }
+
+    while set.join_next().await.is_some() {}
+
+    info!("prop-grid-rs shutting down");
+    Ok(())
+}
+
+async fn worker_loop(
+    worker_id: usize,
+    pool: sqlx::PgPool,
+    client: Arc<HrrrClient>,
+    scores_dir: PathBuf,
+    shutdown: Arc<AtomicBool>,
+) {
     while !shutdown.load(Ordering::Relaxed) {
         match db::claim_next(&pool).await {
             Ok(Some(task)) => {
@@ -62,6 +102,7 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
                 {
                     Ok(stats) => {
                         info!(
+                            worker_id,
                             task_id = %task.id,
                             run_time = %task.run_time,
                             forecast_hour = task.forecast_hour,
@@ -70,13 +111,13 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
                             "chain step done"
                         );
                         if let Err(e) = db::complete(&pool, &task).await {
-                            error!(%task_id, error = %e, "failed to mark task done");
+                            error!(worker_id, %task_id, error = %e, "failed to mark task done");
                         }
                     }
                     Err(e) => {
-                        warn!(%task_id, error = %e, "chain step failed");
+                        warn!(worker_id, %task_id, error = %e, "chain step failed");
                         if let Err(dbe) = db::fail(&pool, task_id, &e.to_string()).await {
-                            error!(%task_id, error = %dbe, "failed to mark task failed");
+                            error!(worker_id, %task_id, error = %dbe, "failed to mark task failed");
                         }
                     }
                 }
@@ -85,14 +126,11 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
                 tokio::time::sleep(IDLE_SLEEP).await;
             }
             Err(e) => {
-                error!(error = %e, "claim_next failed; backing off");
+                error!(worker_id, error = %e, "claim_next failed; backing off");
                 tokio::time::sleep(IDLE_SLEEP).await;
             }
         }
     }
-
-    info!("prop-grid-rs shutting down");
-    Ok(())
 }
 
 #[cfg(unix)]