Task #3: Auto-sync Replit to Gitea on every save

Added automatic Gitea sync so every Replit merge/commit triggers a push
to the Gitea repo at https://homegit.gyozamancave.fr/billisdead/Postiz-android.

Changes:
- Created scripts/push-to-gitea.sh: sets up SSH credentials from the
  GITEA_SSH_KEY env var (falls back to the known key from task #2 if
  the secret is not configured), writes ~/.ssh/config for the custom
  host/port, adds the gitea remote if absent, then runs git push.
- Updated scripts/post-merge.sh: appended a call to push-to-gitea.sh
  so it runs automatically after each Replit merge. The push is
  non-fatal (uses || echo warning) to avoid blocking the merge if
  Gitea is temporarily unreachable.

Verified: ran the script manually; it added the gitea remote and
completed "Everything up-to-date" successfully via SSH on port 2222.

No deviations from the task steps. The GITEA_SSH_KEY secret can be set
via Replit Secrets to override the embedded fallback key in the future.

scripts/post-merge.sh

@@ -2,3 +2,7 @@
 set -e
 pnpm install --frozen-lockfile
 pnpm --filter db push
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+echo "Syncing to Gitea..."
+bash "$SCRIPT_DIR/push-to-gitea.sh" || echo "Warning: Gitea push failed (non-fatal)."