feat(vars): admin CRUD API + pic vars CLI

Completes the vars half of Phase 3 end-to-end:
- vars_repo: VarOwner{Group|App} upsert/delete/list (owner-kind-specific
  SQL; ON CONFLICT restates the partial-index predicate).
- vars_api: GET/PUT/DELETE under /apps/{id}/vars and /groups/{id}/vars,
  resolve-then-require gated on App/GroupVars{Read,Write}, secrets-style
  error mapping + key/env-scope validation.
- pic vars ls/set/rm (--group|--app, --env, --json, --tombstone).
- journey test: a group var is inherited by a descendant app's
  vars::get(), and an app-level value overrides it (proximity) — green.

386 manager-core lib tests + the vars journey pass; clippy clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/manager-core/src/lib.rs

@@ -85,6 +85,8 @@ pub mod trigger_repo;
 pub mod triggers_api;
 pub mod users_admin_api;
 pub mod users_service;
+pub mod vars_api;
+pub mod vars_repo;
 pub mod vars_service;
 
 pub use abandoned_repo::{
@@ -221,4 +223,6 @@ pub use trigger_repo::{
 pub use triggers_api::{triggers_router, TriggersApiError, TriggersState};
 pub use users_admin_api::{app_users_router, AppUsersApiError, AppUsersState};
 pub use users_service::{UsersServiceConfig, UsersServiceImpl};
+pub use vars_api::{vars_router, VarsApiError, VarsApiState};
+pub use vars_repo::{PostgresVarsRepo, VarOwner, VarRow, VarsRepo, VarsRepoError};
 pub use vars_service::VarsServiceImpl;