feat(mcpd): pluggable SecretBackend abstraction + OpenBao driver + migrate

Why: API keys live in Postgres as plaintext JSON. A DB read exposes every
credential in the system. Before centralising more secrets (LLM keys, etc.)
we want to be able to point at an external KV store and drop DB access to
sensitive rows.

New model:
- `SecretBackend` resource (CRUD + isDefault invariant) owns how a secret is
  stored. `Secret` gains `backendId` FK and `externalRef`. Reads/writes
  dispatch through a driver.
- `plaintext` driver (near-noop, uses existing Secret.data column) is seeded
  as the `default` row at startup. Acts as trust root / bootstrap.
- `openbao` driver (also HashiCorp Vault KV v2 compatible) talks plain HTTP,
  no SDK dependency. Auth via static token pulled from a plaintext-backed
  `Secret` through the injected SecretRefResolver. Caches resolved token.
- `SecretMigrateService` moves secrets one-at-a-time: read → write dest →
  flip row → best-effort source delete. Interrupted runs are idempotent
  (skips secrets already on destination).

CLI surface:
- `mcpctl create|get|describe|delete secretbackend` + `--default` on create.
- `mcpctl migrate secrets --from X --to Y [--names a,b] [--keep-source] [--dry-run]`
- `apply -f` round-trips secretbackends (yaml/json multi-doc + grouped).
- RBAC: `secretbackends` resource + `run:migrate-secrets` operation.
- Fish + bash completions regenerated.

docs/secret-backends.md covers the OpenBao policy, chicken-and-egg auth flow,
and the migration semantics.

Broke the circular dep (OpenBao needs SecretService to resolve its own token,
SecretService needs SecretBackendService) with a deferred-resolver bridge in
mcpd startup. 11 new driver unit tests; existing env-resolver/secret-route/
backup tests updated for the new service signatures. Full suite: 1792/1792.

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

completions/mcpctl.bash

@@ -5,11 +5,11 @@ _mcpctl() {
   local cur prev words cword
   _init_completion || return
 
-  local commands="status login logout config get describe delete logs create edit apply patch backup approve console cache test"
+  local commands="status login logout config get describe delete logs create edit apply patch backup approve console cache test migrate"
   local project_commands="get describe delete logs create edit attach-server detach-server"
   local global_opts="-v --version --daemon-url --direct -p --project -h --help"
-  local resources="servers instances secrets templates projects users groups rbac prompts promptrequests serverattachments proxymodels all"
-  local resource_aliases="servers instances secrets templates projects users groups rbac prompts promptrequests serverattachments proxymodels all server srv instance inst secret sec template tpl project proj user group rbac-definition rbac-binding prompt promptrequest pr serverattachment sa proxymodel pm"
+  local resources="servers instances secrets secretbackends templates projects users groups rbac prompts promptrequests serverattachments proxymodels all"
+  local resource_aliases="servers instances secrets secretbackends templates projects users groups rbac prompts promptrequests serverattachments proxymodels all server srv instance inst secret sec secretbackend sb template tpl project proj user group rbac-definition rbac-binding prompt promptrequest pr serverattachment sa proxymodel pm"
 
   # Check if --project/-p was given
   local has_project=false
@@ -175,7 +175,7 @@ _mcpctl() {
     create)
       local create_sub=$(_mcpctl_get_subcmd $subcmd_pos)
       if [[ -z "$create_sub" ]]; then
-        COMPREPLY=($(compgen -W "server secret project user group rbac mcptoken prompt serverattachment promptrequest help" -- "$cur"))
+        COMPREPLY=($(compgen -W "server secret secretbackend project user group rbac mcptoken prompt serverattachment promptrequest help" -- "$cur"))
       else
         case "$create_sub" in
           server)
@@ -184,6 +184,9 @@ _mcpctl() {
           secret)
             COMPREPLY=($(compgen -W "--data --force -h --help" -- "$cur"))
             ;;
+          secretbackend)
+            COMPREPLY=($(compgen -W "--type --description --default --url --namespace --mount --path-prefix --token-secret --config --force -h --help" -- "$cur"))
+            ;;
           project)
             COMPREPLY=($(compgen -W "-d --description --proxy-model --prompt --gated --no-gated --server --force -h --help" -- "$cur"))
             ;;
@@ -329,6 +332,21 @@ _mcpctl() {
         esac
       fi
       return ;;
+    migrate)
+      local migrate_sub=$(_mcpctl_get_subcmd $subcmd_pos)
+      if [[ -z "$migrate_sub" ]]; then
+        COMPREPLY=($(compgen -W "secrets help" -- "$cur"))
+      else
+        case "$migrate_sub" in
+          secrets)
+            COMPREPLY=($(compgen -W "--from --to --names --keep-source --dry-run -h --help" -- "$cur"))
+            ;;
+          *)
+            COMPREPLY=($(compgen -W "-h --help" -- "$cur"))
+            ;;
+        esac
+      fi
+      return ;;
     help)
       COMPREPLY=($(compgen -W "$commands" -- "$cur"))
       return ;;