make so-yaml PG-canonical and add pillar-change reactor stack

Two coupled changes that together let so_pillar.* be the canonical
config store, with config edits driving service reloads automatically:

so-yaml PG-canonical mode
- Adds /opt/so/conf/so-yaml/mode (and SO_YAML_BACKEND env override) with
  three values: dual (legacy), postgres (PG-only for managed paths),
  disk (emergency rollback). Bootstrap files (secrets.sls, ca/init.sls,
  *.nodes.sls, top.sls, ...) stay disk-only regardless via the existing
  SkipPath allowlist in so_yaml_postgres.locate.
- loadYaml/writeYaml/purgeFile now route to so_pillar.* in postgres
  mode: replace/add/get all read+write the database with no disk file
  ever appearing. PG failure is fatal in postgres mode (no silent
  fallback); dual mode preserves the prior best-effort mirror.
- so_yaml_postgres gains read_yaml(path), is_pg_managed(path), and
  is_enabled() so so-yaml can answer "is this path PG-managed and is
  PG up" without reaching into private helpers.
- schema_pillar.sls writes /opt/so/conf/so-yaml/mode = postgres after
  the importer succeeds, so flipping postgres:so_pillar:enabled flips
  so-yaml's behavior in lockstep with the schema being live.

pg_notify-driven change fan-out
- 008_change_notify.sql adds so_pillar.change_queue + an AFTER trigger
  on pillar_entry that enqueues the locator and pg_notifies
  'so_pillar_change'. Queue is drained at-least-once so engine restarts
  don't lose events; pg_notify is just the wakeup signal.
- New salt-master engine pg_notify_pillar.py LISTENs on the channel,
  drains the queue with FOR UPDATE SKIP LOCKED, debounces bursts, and
  fires 'so/pillar/changed' events grouped by (scope, role, minion).
- Reactor so_pillar_changed.sls catches the tag and dispatches to
  orch.so_pillar_reload, which carries a DISPATCH map of pillar-path
  prefix -> (state sls, role grain set) so adding a new service to
  the auto-reload list is a one-line edit instead of a new reactor.
- Engine + reactor wiring is gated on the same postgres:so_pillar:enabled
  flag as the schema and ext_pillar config so the whole stack flips
  on/off together.

Tests: 21 new cases (112 total, all passing) covering mode resolution,
PG-managed detection, and PG-canonical read/write/purge routing with
the PG client stubbed.

salt/postgres/files/schema/pillar/006_rls.sql

@@ -37,6 +37,16 @@ GRANT SELECT ON so_pillar.v_pillar_global,
     TO so_pillar_master;
 GRANT EXECUTE ON FUNCTION so_pillar.fn_pillar_secrets(text) TO so_pillar_master;
 
+-- Engine reads + drains the change queue from the salt-master process. It
+-- needs SELECT to find unprocessed rows and UPDATE to mark them processed.
+-- The queue contains only locator metadata (no pillar data), so the master
+-- role's existing privilege footprint is unchanged in practice.
+GRANT SELECT, UPDATE ON so_pillar.change_queue TO so_pillar_master;
+GRANT USAGE ON SEQUENCE so_pillar.change_queue_id_seq TO so_pillar_master;
+-- Writer needs INSERT (the trigger runs as table owner, so this is just for
+-- direct testing / manual replays from psql).
+GRANT INSERT ON so_pillar.change_queue TO so_pillar_writer;
+
 -- Writer needs CRUD on pillar_entry/minion/role_member plus access to seed tables.
 GRANT SELECT, INSERT, UPDATE, DELETE
     ON so_pillar.pillar_entry,

salt/postgres/files/schema/pillar/008_change_notify.sql

@@ -0,0 +1,77 @@
+-- pg_notify-driven change fan-out for so_pillar.pillar_entry.
+--
+-- Two layers:
+--   1. so_pillar.change_queue          — durable, drained by the salt-master
+--                                        engine. Survives engine downtime,
+--                                        de-duplicated by id, processed once.
+--   2. pg_notify('so_pillar_change')   — wakeup signal. Payload is the
+--                                        change_queue row id and locator
+--                                        (no secret data — channels are
+--                                        snoopable by anyone with LISTEN).
+--
+-- The salt-master engine LISTENs on the channel for low-latency wakeup,
+-- then SELECTs unprocessed change_queue rows so a missed notification
+-- (engine restart, network blip) self-heals on the next event.
+
+CREATE TABLE IF NOT EXISTS so_pillar.change_queue (
+    id            bigserial PRIMARY KEY,
+    scope         text        NOT NULL,
+    role_name     text,
+    minion_id     text,
+    pillar_path   text        NOT NULL,
+    op            text        NOT NULL CHECK (op IN ('INSERT','UPDATE','DELETE')),
+    enqueued_at   timestamptz NOT NULL DEFAULT now(),
+    processed_at  timestamptz
+);
+
+-- Hot index for the engine's drain query.
+CREATE INDEX IF NOT EXISTS ix_change_queue_unprocessed
+    ON so_pillar.change_queue (id)
+    WHERE processed_at IS NULL;
+
+-- Retention index: pg_cron job in 007 sweeps processed rows older than 7d.
+CREATE INDEX IF NOT EXISTS ix_change_queue_processed_at
+    ON so_pillar.change_queue (processed_at)
+    WHERE processed_at IS NOT NULL;
+
+CREATE OR REPLACE FUNCTION so_pillar.fn_pillar_entry_notify()
+    RETURNS trigger
+    LANGUAGE plpgsql
+AS $$
+DECLARE
+    v_row record;
+    v_id  bigint;
+BEGIN
+    IF TG_OP = 'DELETE' THEN
+        v_row := OLD;
+    ELSE
+        v_row := NEW;
+    END IF;
+
+    INSERT INTO so_pillar.change_queue
+        (scope, role_name, minion_id, pillar_path, op)
+    VALUES
+        (v_row.scope, v_row.role_name, v_row.minion_id, v_row.pillar_path, TG_OP)
+    RETURNING id INTO v_id;
+
+    -- Payload is the queue id + locator only. Engine joins back to
+    -- pillar_entry if it needs the data — keeps secrets off the wire.
+    PERFORM pg_notify('so_pillar_change', json_build_object(
+        'queue_id',    v_id,
+        'scope',       v_row.scope,
+        'role_name',   v_row.role_name,
+        'minion_id',   v_row.minion_id,
+        'pillar_path', v_row.pillar_path,
+        'op',          TG_OP
+    )::text);
+
+    RETURN NULL;
+END;
+$$;
+
+DROP TRIGGER IF EXISTS tg_pillar_entry_notify ON so_pillar.pillar_entry;
+CREATE TRIGGER tg_pillar_entry_notify
+    AFTER INSERT OR UPDATE OR DELETE
+    ON so_pillar.pillar_entry
+    FOR EACH ROW
+    EXECUTE FUNCTION so_pillar.fn_pillar_entry_notify();