feat(stores)!: use backend storage references instead of configs (#3697)

**This PR changes configurations in a backward incompatible way.**

Run configs today repeat full SQLite/Postgres snippets everywhere a
store is needed, which means duplicated credentials, extra connection
pools, and lots of drift between files. This PR introduces named storage
backends so the stack and providers can share a single catalog and
reference those backends by name.

## Key Changes

- Add `storage.backends` to `StackRunConfig`, register each KV/SQL
backend once at startup, and validate that references point to the right
family.
- Move server stores under `storage.stores` with lightweight references
(backend + namespace/table) instead of full configs.
- Update every provider/config/doc to use the new reference style;
docs/codegen now surface the simplified YAML.

## Migration

Before:
```yaml
metadata_store:
  type: sqlite
  db_path: ~/.llama/distributions/foo/registry.db
inference_store:
  type: postgres
  host: ${env.POSTGRES_HOST}
  port: ${env.POSTGRES_PORT}
  db: ${env.POSTGRES_DB}
  user: ${env.POSTGRES_USER}
  password: ${env.POSTGRES_PASSWORD}
conversations_store:
  type: postgres
  host: ${env.POSTGRES_HOST}
  port: ${env.POSTGRES_PORT}
  db: ${env.POSTGRES_DB}
  user: ${env.POSTGRES_USER}
  password: ${env.POSTGRES_PASSWORD}
```

After:
```yaml
storage:
  backends:
    kv_default:
      type: kv_sqlite
      db_path: ~/.llama/distributions/foo/kvstore.db
    sql_default:
      type: sql_postgres
      host: ${env.POSTGRES_HOST}
      port: ${env.POSTGRES_PORT}
      db: ${env.POSTGRES_DB}
      user: ${env.POSTGRES_USER}
      password: ${env.POSTGRES_PASSWORD}
  stores:
    metadata:
      backend: kv_default
      namespace: registry
    inference:
      backend: sql_default
      table_name: inference_store
      max_write_queue_size: 10000
      num_writers: 4
    conversations:
      backend: sql_default
      table_name: openai_conversations
```

Provider configs follow the same pattern—for example, a Chroma vector
adapter switches from:

```yaml
providers:
  vector_io:
  - provider_id: chromadb
    provider_type: remote::chromadb
    config:
      url: ${env.CHROMADB_URL}
      kvstore:
        type: sqlite
        db_path: ~/.llama/distributions/foo/chroma.db
```

to:

```yaml
providers:
  vector_io:
  - provider_id: chromadb
    provider_type: remote::chromadb
    config:
      url: ${env.CHROMADB_URL}
      persistence:
        backend: kv_default
        namespace: vector_io::chroma_remote
```

Once the backends are declared, everything else just points at them, so
rotating credentials or swapping to Postgres happens in one place and
the stack reuses a single connection pool.

tests/unit/cli/test_stack_config.py

@@ -23,6 +23,27 @@ def config_with_image_name_int():
         image_name: 1234
         apis_to_serve: []
         built_at: {datetime.now().isoformat()}
+        storage:
+          backends:
+            kv_default:
+              type: kv_sqlite
+              db_path: /tmp/test_kv.db
+            sql_default:
+              type: sql_sqlite
+              db_path: /tmp/test_sql.db
+          stores:
+            metadata:
+              backend: kv_default
+              namespace: metadata
+            inference:
+              backend: sql_default
+              table_name: inference
+            conversations:
+              backend: sql_default
+              table_name: conversations
+            responses:
+              backend: sql_default
+              table_name: responses
         providers:
           inference:
             - provider_id: provider1
@@ -54,6 +75,27 @@ def up_to_date_config():
         image_name: foo
         apis_to_serve: []
         built_at: {datetime.now().isoformat()}
+        storage:
+          backends:
+            kv_default:
+              type: kv_sqlite
+              db_path: /tmp/test_kv.db
+            sql_default:
+              type: sql_sqlite
+              db_path: /tmp/test_sql.db
+          stores:
+            metadata:
+              backend: kv_default
+              namespace: metadata
+            inference:
+              backend: sql_default
+              table_name: inference
+            conversations:
+              backend: sql_default
+              table_name: conversations
+            responses:
+              backend: sql_default
+              table_name: responses
         providers:
           inference:
             - provider_id: provider1