Make swapfile a default for every install, not just Asterisk droplets

Extracts the swapfile logic out of services/asterisk.sh (previously
DigitalOcean-droplet-gated) into lib/common.sh's ensure_swapfile() —
provider detection was never really the point, the actual condition
that matters is "modest RAM, no swap yet," which applies just as much
to a non-DO VPS running several Docker services at once as it did to a
single-purpose droplet.

- lib/common.sh: new ensure_swapfile(), same fallocate/mkswap/fstab/
  swappiness logic as before, threshold raised from 2048MB to 4096MB
  (a 4GB box running a full service stack is exactly the case that
  motivated this change — the old threshold would have skipped it).
- services/base.sh: calls it unconditionally so every install gets the
  same check regardless of which other services get chosen.
- services/asterisk.sh: swapfile call is no longer gated behind
  IS_DO — calls the shared helper directly. Kept a standalone-mode
  stub (same pattern as this file's other stubbed helpers) so
  `sudo bash asterisk.sh` with no base.sh in the picture still gets
  it. Idempotent either way: a box that already has swap, or already
  got it from base.sh earlier in the same run, no-ops immediately.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TBtExJcqxnokyZZKmphdug

services/base.sh

@@ -13,6 +13,7 @@ install_base() {
         echo "[DRY-RUN] Would install Docker CE + Compose plugin"
         echo "[DRY-RUN] Would detect an NVIDIA GPU and offer to install the driver"
         echo "    + NVIDIA Container Toolkit (for GPU-accelerated Docker services)"
+        echo "[DRY-RUN] Would add a swapfile if RAM <= 4096MB and none exists"
         echo "[DRY-RUN] Would install/configure openssh-server"
         echo "[DRY-RUN] Would offer SSH key import from GitHub/Launchpad"
         echo "[DRY-RUN] Would offer to disable SSH password auth"
@@ -38,6 +39,9 @@ install_base() {
     # ── Docker ───────────────────────────────────────────────────────────────
     require_docker || log_warning "Docker install failed — will retry after base setup"
 
+    # ── Swapfile — default for every install, not just Asterisk droplets ────
+    ensure_swapfile
+
     # ── NVIDIA GPU (driver + container toolkit) ─────────────────────────────
     _base_setup_nvidia_gpu