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/go.mod

@@ -37,23 +37,29 @@ require (
 	github.com/dgryski/go-rendezvous v0.0.0-20200823014737-9f7001d12a5f // indirect
 	github.com/distribution/reference v0.6.0 // indirect
 	github.com/docker/go-units v0.5.0 // indirect
+	github.com/dustin/go-humanize v1.0.1 // indirect
 	github.com/felixge/httpsnoop v1.0.4 // indirect
 	github.com/fsnotify/fsnotify v1.9.0 // indirect
 	github.com/gabriel-vasile/mimetype v1.4.12 // indirect
 	github.com/gin-contrib/sse v0.1.0 // indirect
+	github.com/go-ini/ini v1.67.0 // indirect
 	github.com/go-logr/logr v1.4.3 // indirect
 	github.com/go-logr/stdr v1.2.2 // indirect
 	github.com/go-playground/locales v0.14.1 // indirect
 	github.com/go-playground/universal-translator v0.18.1 // indirect
 	github.com/go-viper/mapstructure/v2 v2.4.0 // indirect
-	github.com/goccy/go-json v0.10.2 // indirect
+	github.com/goccy/go-json v0.10.5 // indirect
 	github.com/google/jsonschema-go v0.4.3 // indirect
 	github.com/inconshreveable/mousetrap v1.1.0 // indirect
 	github.com/json-iterator/go v1.1.12 // indirect
-	github.com/klauspost/cpuid/v2 v2.2.4 // indirect
+	github.com/klauspost/compress v1.18.0 // indirect
+	github.com/klauspost/cpuid/v2 v2.2.11 // indirect
 	github.com/leodido/go-urn v1.4.0 // indirect
 	github.com/mattn/go-isatty v0.0.20 // indirect
 	github.com/mfridman/interpolate v0.0.2 // indirect
+	github.com/minio/crc64nvme v1.0.2 // indirect
+	github.com/minio/md5-simd v1.1.2 // indirect
+	github.com/minio/minio-go/v7 v7.0.95 // indirect
 	github.com/moby/docker-image-spec v1.3.1 // indirect
 	github.com/moby/sys/atomicwriter v0.1.0 // indirect
 	github.com/moby/term v0.5.2 // indirect
@@ -63,8 +69,10 @@ require (
 	github.com/opencontainers/go-digest v1.0.0 // indirect
 	github.com/opencontainers/image-spec v1.1.1 // indirect
 	github.com/pelletier/go-toml/v2 v2.2.4 // indirect
+	github.com/philhofer/fwd v1.2.0 // indirect
 	github.com/pkg/errors v0.9.1 // indirect
 	github.com/pmezard/go-difflib v1.0.0 // indirect
+	github.com/rs/xid v1.6.0 // indirect
 	github.com/sagikazarmark/locafero v0.11.0 // indirect
 	github.com/segmentio/asm v1.2.0 // indirect
 	github.com/segmentio/encoding v0.5.4 // indirect
@@ -74,6 +82,7 @@ require (
 	github.com/spf13/cast v1.10.0 // indirect
 	github.com/spf13/pflag v1.0.10 // indirect
 	github.com/subosito/gotenv v1.6.0 // indirect
+	github.com/tinylib/msgp v1.3.0 // indirect
 	github.com/twitchyliquid64/golang-asm v0.15.1 // indirect
 	github.com/ugorji/go/codec v1.2.11 // indirect
 	github.com/yosida95/uritemplate/v3 v3.0.2 // indirect