feat(databases): S3-compatible backup targets — offsite archives + remote restore

Phase D3. Managed databases can ship backup archives to a configured
S3-compatible destination (AWS S3, MinIO, Garage, …) while the local
copy remains intact.

- backup_targets table: owner-scoped endpoint/bucket/region/prefix +
  encrypted access/secret keys (enc:v1:, never returned, has_credentials)
- create/update probe bucket access before storing; empty keys on update
  keep stored credentials; delete refused (409) while referenced
- databases.backup_target_id assigns a target; external DBs rejected
- backup pipeline uploads via a holder container over docker cp streams;
  remote failure keeps the local archive (logged), row gets remote:true
- restore prefers the local archive; on a missing local copy it downloads
  the remote object into the backup volume first
- holder containers must be started before archive ops — docker cp does
  not resolve volume mounts on never-started (Created) containers, and
  removing the source mid-stream stalls the reader without EOF
- CLI: backup-targets list|add|update|remove|test + databases update
  --backup-target; MCP: 5 new tools; Settings UI card + per-database
  target picker + offsite badge on backups; OpenAPI + llms.txt + CLI.md

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

app/backend/migrations_goose/20261015000000_backup_targets.sql

@@ -0,0 +1,34 @@
+-- +goose Up
+-- +goose StatementBegin
+CREATE TABLE IF NOT EXISTS backup_targets (
+    id VARCHAR(255) PRIMARY KEY,
+    user_id VARCHAR(255) NOT NULL,
+    name VARCHAR(255) NOT NULL,
+    endpoint VARCHAR(512) NOT NULL,
+    bucket VARCHAR(255) NOT NULL,
+    region VARCHAR(100) NOT NULL DEFAULT '',
+    prefix VARCHAR(255) NOT NULL DEFAULT '',
+    access_key TEXT NOT NULL DEFAULT '',
+    secret_key TEXT NOT NULL DEFAULT '',
+    use_tls BOOLEAN NOT NULL DEFAULT true,
+    created_at TIMESTAMP WITH TIME ZONE DEFAULT NOW(),
+    updated_at TIMESTAMP WITH TIME ZONE DEFAULT NOW()
+);
+
+ALTER TABLE database_services
+    ADD COLUMN IF NOT EXISTS backup_target_id VARCHAR(255);
+
+ALTER TABLE database_backups
+    ADD COLUMN IF NOT EXISTS remote_key VARCHAR(512);
+-- +goose StatementEnd
+
+-- +goose Down
+-- +goose StatementBegin
+ALTER TABLE database_backups
+    DROP COLUMN IF EXISTS remote_key;
+
+ALTER TABLE database_services
+    DROP COLUMN IF EXISTS backup_target_id;
+
+DROP TABLE IF EXISTS backup_targets;
+-- +goose StatementEnd