postgresql: drop per-database autovacuum ALTERs (rejected by Postgres)

Follow-up to the previous two commits: the ALTER DATABASE ... SET
autovacuum_* commands fail at boot with

  ERROR: parameter "autovacuum_vacuum_scale_factor" cannot be changed now

and take matrix-synapse-db-tune.service (and nextcloud-db-init.service)
down with them.

Root cause: ALTER DATABASE/ROLE ... SET validates through
set_config_option() with an interactive context, and guc.c rejects any
PGC_SIGHUP parameter set that way. All autovacuum_* GUCs are
SIGHUP-context, so per-database scoping is impossible for them — only
USERSET-level parameters (e.g. work_mem, statement_timeout) can be set
per-database.

Nothing is lost: the same values are already set cluster-wide in
configuration.nix, which covers both nextclouddb and matrix-synapse.
Remove the ALTERs from nextcloud-db-init and delete the now-purposeless
matrix-synapse-db-tune service.

modules/nextcloud.nix

@@ -60,11 +60,11 @@ lib.mkIf config.sovran_systemsOS.services.nextcloud {
         psql -U postgres -c "CREATE DATABASE nextclouddb WITH OWNER ncusr TEMPLATE template0 LC_COLLATE = 'C' LC_CTYPE = 'C';"
       fi
 
-      # Per-database autovacuum, scoped to nextclouddb only.
+      # NOTE: autovacuum GUCs are SIGHUP-context, so they cannot be set
-      # The shared matrix-synapse DB keeps the milder cluster defaults.
+      # per-database — ALTER DATABASE ... SET rejects them with
-      # Fixes Nextcloud 35 pg.dead_tuples warning. Idempotent.
+      # 'parameter "..." cannot be changed now'. They are set
-      psql -U postgres -d nextclouddb -c "ALTER DATABASE nextclouddb SET autovacuum_vacuum_scale_factor = '0.05';"
+      # cluster-wide in configuration.nix instead, which already covers
-      psql -U postgres -d nextclouddb -c "ALTER DATABASE nextclouddb SET autovacuum_analyze_scale_factor = '0.025';"
+      # both nextclouddb and matrix-synapse.
     '';
   };
 

modules/synapse.nix

@@ -60,35 +60,6 @@ lib.mkIf config.sovran_systemsOS.services.synapse {
     '';
   };
 
-  # ── Per-database Postgres tuning (matrix-synapse only) ─────
-  # Mirrors the nextclouddb tuning: Synapse's state and event tables are
-  # write-heavy and bloat fast under stock autovacuum. Scoped via ALTER
-  # DATABASE so each DB gets what suits it. Idempotent.
-  systemd.services.matrix-synapse-db-tune = {
-    description = "Apply per-database Postgres tuning for Matrix Synapse";
-    after = [ "postgresql.service" ];
-    requires = [ "postgresql.service" ];
-    before = [ "matrix-synapse.service" ];
-    wantedBy = [ "multi-user.target" ];
-    serviceConfig = {
-      Type = "oneshot";
-      RemainAfterExit = true;
-    };
-    path = [ config.services.postgresql.package pkgs.coreutils ];
-    script = ''
-      set -euo pipefail
-      # Wait for ensureDatabases to have created the DB on first boot.
-      for i in $(seq 1 30); do
-        if psql -U postgres -lqt | cut -d \| -f 1 | grep -qw "matrix-synapse"; then
-          break
-        fi
-        sleep 2
-      done
-      psql -U postgres -d matrix-synapse -c "ALTER DATABASE \"matrix-synapse\" SET autovacuum_vacuum_scale_factor = '0.05';"
-      psql -U postgres -d matrix-synapse -c "ALTER DATABASE \"matrix-synapse\" SET autovacuum_analyze_scale_factor = '0.025';"
-    '';
-  };
-
   # ── Generate runtime config from domain files ───────────────
   systemd.services.matrix-synapse-runtime-config = {
     description = "Generate Synapse runtime config from domain files";