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/theme.ts

@@ -27,7 +27,7 @@ export function applyTheme(t: ThemePrefs): void {
   el.style.setProperty("--font-scale", String(t.fontScale));
 }
 
-const KEY = "subfeed.theme";
+const KEY = "siftlode.theme";
 
 export function loadLocalTheme(): ThemePrefs {
   try {