fix(infra): raise Prometheus retention.size 1GB -> 6GB

Prometheus size-based retention counts the WAL toward its budget. With
retention.size=1GB and a stuck/corrupt WAL that had grown to 2.1GB, the
total was permanently over budget, so Prometheus deleted every
freshly-compacted block the moment it was written. Net effect: only the
in-memory ~2h head block was ever queryable — metrics appeared to "start
this morning" no matter when you looked, and last night's Caddy 502
spike (planner /api/overpass upstream stall, 18:33-18:34 UTC) was
unobservable in Prometheus (logs survived in Loki).

The corrupt WAL was cleared out-of-band on the flagship. This raises the
size backstop to 6GB so compacted blocks survive to the intended 15d
time-based retention. Disk is 38G (43% used), so 6GB is safe headroom.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

infrastructure/docker-compose.yml

@@ -201,7 +201,13 @@ services:
     command:
       - "--config.file=/etc/prometheus/prometheus.yml"
       - "--storage.tsdb.retention.time=15d"
-      - "--storage.tsdb.retention.size=1GB"
+      # Size-based retention is a disk-safety backstop, not the primary
+      # policy (time=15d is). It counts the WAL toward the budget, so a
+      # too-small cap makes Prometheus delete freshly-compacted blocks on
+      # creation — which it did at 1GB, leaving only the ~2h head block
+      # queryable (a stuck/corrupt WAL had bloated to 2.1GB). 6GB holds
+      # ~15d of metrics at current cardinality with headroom; disk is 38G.
+      - "--storage.tsdb.retention.size=6GB"
 
   loki:
     image: grafana/loki:latest