chore: upgrade to 7.3.0+v1.154.0

- matrixdotorg/synapse: v1.149.1 -> v1.154.0
- pgautoupgrade/pgautoupgrade: 17-alpine -> 18-alpine (+ PGDATA pin for in-place upgrade)
- ghcr.io/element-hq/matrix-authentication-service: 1.14.0 -> 1.18.0
- nginx: 1.29.6 -> 1.31.1
- discorddb postgres: 13-alpine -> 16-alpine (conservative bump; manual dump/restore required)

Deferred to separate PR:
- mautrix/signal v0.8.7 -> v26.02.2 (CalVer; requires signaldb dump/restore)
- mautrix/telegram v0.15.3 -> v0.2605.0 (CalVer; requires telegramdb dump/restore)
- signaldb/telegramdb postgres: 13-alpine (deferred with bridges)

compose.discord.yml

@@ -30,7 +30,7 @@ services:
       - internal
 
   discorddb:
-    image: postgres:13-alpine
+    image: postgres:16-alpine
     secrets:
       - discord_db_password
     environment:

compose.yml

@@ -114,7 +114,7 @@ services:
       restart_policy:
         condition: on-failure
       labels:
-        - "coop-cloud.${STACK_NAME}.version=7.2.0+v1.154.0"
+        - "coop-cloud.${STACK_NAME}.version=7.3.0+v1.154.0"
         - "coop-cloud.${STACK_NAME}.timeout=${TIMEOUT}"
     healthcheck:
       test: ["CMD", "curl", "-f", "http://localhost:8008/health"]
@@ -128,9 +128,9 @@ services:
     secrets:
       - db_password
     environment:
-      # postgres:18 relocated the default PGDATA to /var/lib/postgresql/18/docker.
+      # postgres:18 relocated default PGDATA to /var/lib/postgresql/18/docker.
-      # Pin it to the legacy path so pgautoupgrade finds the existing cluster on the
+      # Pin to legacy path so pgautoupgrade finds the existing cluster and
-      # mounted volume and upgrades it in place, instead of init'ing an empty one.
+      # upgrades in place instead of init-ing an empty one.
       - PGDATA=/var/lib/postgresql/data
       - LC_COLLATE=C
       - LC_CTYPE=C