Initial commit: backup + mirror automation for self-hosted Gitea

Includes:
- gitea-backups/bin/backup.sh (per-push bundle + DB snapshot to local + S3)
- gitea-backups/bin/install-hooks.sh (idempotent post-receive shim installer)
- gitea-backups/bin/retention.sh (count-based retention: keep newest 7 dates)
- gitea-mirror/bin/auto-mirror.sh (Gitea -> GitHub push mirror automation,
  hardened against Gitea outages)
- crontab.txt (reference for the 3 cron entries)
- README.md (architecture, layout, bootstrap)

gitea-backups/bin/install-hooks.sh

@@ -0,0 +1,35 @@
+#!/usr/bin/env bash
+# Install / refresh the per-repo backup shim in every Gitea repo.
+# Idempotent: safe to run anytime, including via cron.
+# Usage: ./install-hooks.sh [--quiet]
+
+set -euo pipefail
+
+GITEA_REPOS="/home/ubuntu/gitea/data/gitea-repositories"
+SHIM_NAME="zzz-backup"
+QUIET=0
+[[ "${1:-}" == "--quiet" ]] && QUIET=1
+
+shim_content='#!/usr/bin/env bash
+exec /home/ubuntu/gitea-backups/bin/backup.sh
+'
+
+installed=0
+already=0
+for repo in "$GITEA_REPOS"/*/*.git; do
+    [[ -d "$repo" ]] || continue
+    hook_dir="$repo/hooks/post-receive.d"
+    mkdir -p "$hook_dir"
+    shim="$hook_dir/$SHIM_NAME"
+    if [[ -f "$shim" ]] && diff -q <(printf '%s' "$shim_content") "$shim" >/dev/null 2>&1; then
+        already=$((already + 1))
+    else
+        printf '%s' "$shim_content" > "$shim"
+        chmod +x "$shim"
+        installed=$((installed + 1))
+        [[ $QUIET -eq 0 ]] && echo "installed: $repo"
+    fi
+done
+
+[[ $QUIET -eq 0 ]] && echo "done — installed/updated: $installed, already-current: $already"
+exit 0