refactor: absorb _backup and _update into stacks.sh

- Inline update logic (pull → compare digests → up -d → prune → notify)
- Inline backup logic with dynamic Postgres detection: any running
  <stack>_db container is dumped using the <stack>/<stack> convention
- Systemd unit files are now generated on `install` from embedded
  heredocs pointing at stacks.sh itself — no external scripts needed
- Root .env (WEBHOOK_URL, RESTIC_REPOSITORY, RESTIC_PASSWORD) replaces
  the per-service .env files in _backup/ and _update/
- Remove _backup/ and _update/ directories entirely
- Update README accordingly

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

_update/update.sh

@@ -1,62 +0,0 @@
-#!/usr/bin/env bash
-set -euo pipefail
-
-SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
-STACKS_DIR="$(dirname "$SCRIPT_DIR")"
-LOG_FILE="$SCRIPT_DIR/update.log"
-
-set -a; source "$SCRIPT_DIR/.env"; set +a
-
-log()    { echo "[$(date '+%Y-%m-%d %H:%M:%S')] $*" | tee -a "$LOG_FILE"; }
-notify() {
-  local color="$1" text="$2"
-  curl -sf -o /dev/null -X POST "$WEBHOOK_URL" \
-    -H 'Content-Type: application/json' \
-    -d "{\"message\":\"$text\",\"color\":\"$color\"}"
-}
-
-: > "$LOG_FILE"
-log "Starting image update check"
-
-updated=(); failed=(); checked=0
-
-for stack_dir in "$STACKS_DIR"/*/; do
-  stack=$(basename "$stack_dir")
-  [[ "$stack" == _* ]] && continue
-  [[ ! -f "$stack_dir/compose.yml" ]] && continue
-
-  log "Checking $stack"
-  cd "$stack_dir"
-  checked=$((checked + 1))
-
-  before=$(docker compose images -q 2>/dev/null | sort)
-  docker compose pull 2>&1 | tee -a "$LOG_FILE"
-  after=$(docker compose images -q 2>/dev/null | sort)
-
-  if [ "$before" != "$after" ]; then
-    log "  Updates found — recreating containers"
-    if docker compose up -d 2>&1 | tee -a "$LOG_FILE"; then
-      updated+=("$stack")
-    else
-      log "  FAILED to recreate $stack"
-      failed+=("$stack")
-    fi
-  else
-    log "  Up to date"
-  fi
-done
-
-log "Pruning dangling images"
-docker image prune -f 2>&1 | tee -a "$LOG_FILE"
-
-date_str=$(date '+%Y-%m-%d %H:%M')
-
-if [ ${#failed[@]} -gt 0 ]; then
-  notify "#cc0000" "❌ **Update failed** — $date_str\nUpdated: ${updated[*]:-none}\nFailed: ${failed[*]}"
-elif [ ${#updated[@]} -gt 0 ]; then
-  notify "#36a64f" "✅ **Updates applied** — $date_str\nStacks updated (${#updated[@]}/$checked): ${updated[*]}"
-else
-  notify "#36a64f" "✅ **All up to date** — $date_str ($checked stacks checked)"
-fi
-
-log "Update check complete"