feat(backup): Off-Site-Upload nach S3 + SFTP

Schutz gegen Box-Total-Loss — lokale Backups in /var/backups/edgeguard
helfen nicht, wenn die Disk stirbt oder die Box brennt. Nach jedem
erfolgreichen lokalen Backup wird die tar.gz an alle aktiven
Off-Site-Ziele hochgeladen.

Migration 0022: backup_remotes (kind=s3|sftp, target_url, settings
JSONB, active, last_upload_at, last_error) + backups.remote_uploads
JSONB (per-Target-Result).

internal/services/backup/remote/:
  - UploadAll() — pro aktivem Target ein Upload, Failures non-fatal
  - S3 via minio-go/v7 — funktioniert mit AWS, MinIO, Backblaze B2,
    Cloudflare R2, Hetzner Object Storage (alle S3-API-kompatibel)
  - SFTP via golang.org/x/crypto/ssh + pkg/sftp. Password + Private-
    Key (OpenSSH, base64-encoded) als Auth. Optional host_key_
    fingerprint-Pinning (SHA256:...); leer = TOFU (unsicher vs MitM,
    OK für initial setup).
  - Test() lädt eine 1KB-Probe + löscht sie wieder — Operator-UI hat
    einen „Verbindung testen"-Button.

backup.Service.RemoteUploader-Interface: nach erfolgreichem
recordSuccess() läuft UploadAll, Results landen in backups.remote_
uploads JSONB. last_upload_at/last_error in backup_remotes pro Target
gepflegt. API + Scheduler injizieren beide den Adapter.

internal/handlers/backup_remotes.go: CRUD + POST /:id/test. Sensitive
Felder (secret_key, password, private_key) werden in GET-Responses
durch ***SET*** maskiert; UpdateChannel merged das zurück damit der
Operator bei Edit ohne Re-Eingabe speichern kann.

UI: Backups-Page jetzt mit Tabs "Sicherungen" + "Off-Site-Ziele".
Tab 2 hat CRUD-Tabelle mit kind-konditionalem Form (S3-Felder oder
SFTP-Felder), Test-Button pro Row, last_upload-Status mit FAIL-Tag
bei Errors.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

go.sum

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@@ -70,6 +83,12 @@ github.com/mfridman/interpolate v0.0.2 h1:pnuTK7MQIxxFz1Gr+rjSIx9u7qVjf5VOoM/u6B
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@@ -80,7 +99,11 @@ github.com/ncruces/go-strftime v1.0.0 h1:HMFp8mLCTPp341M/ZnA4qaf7ZlsbTc+miZjCLOF
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@@ -90,6 +113,8 @@ github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec h1:W09IVJc94
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