chore: rename remaining subfeed references to siftlode

Replace the leftover 'subfeed' name across logger names + log_config,
frontend localStorage keys, Postgres user/db/volume defaults in the
compose files, .env.example, config.py, backup/restore scripts and the
README. Pure rename; no behavioural change. localStorage keys move from
subfeed.* to siftlode.* (one-time UI-state reset is acceptable).

frontend/src/lib/hints.ts

@@ -1,7 +1,7 @@
 // App-wide "hints" toggle: when on, Tooltip components reveal short explanatory
 // captions on hover so the UI is somewhat self-documenting for new users. Experienced
 // users can turn it off in Settings. Persisted to localStorage (+ server preferences).
-const KEY = "subfeed.hints";
+const KEY = "siftlode.hints";
 
 let enabled = load();
 let listeners: Array<() => void> = [];