fix: harden storage semantics (#4118)

Fixes issues in the storage system by guaranteeing immediate durability
for responses and ensuring background writers stay alive. Three related
fixes:

* Responses to the OpenAI-compatible API now write directly to
Postgres/SQLite inside the request instead of detouring through an async
queue that might never drain; this restores the expected
read-after-write behavior and removes the "response not found" races
reported by users.

* The access-control shim was stamping owner_principal/access_attributes
as SQL NULL, which Postgres interprets as non-public rows; fixing it to
use the empty-string/JSON-null pattern means conversations and responses
stored without an authenticated user stay queryable (matching SQLite).

* The inference-store queue remains for batching, but its worker tasks
now start lazily on the live event loop so server startup doesn't cancel
them—writes keep flowing even when the stack is launched via llama stack
run.

Closes #4115 

### Test Plan

Added a matrix entry to test our "base" suite against Postgres as the
store.

src/llama_stack/distributions/starter-gpu/run-with-postgres-store.yaml

@@ -165,20 +165,15 @@ providers:
   - provider_id: meta-reference
     provider_type: inline::meta-reference
     config:
-      persistence_store:
-        type: sql_postgres
-        host: ${env.POSTGRES_HOST:=localhost}
-        port: ${env.POSTGRES_PORT:=5432}
-        db: ${env.POSTGRES_DB:=llamastack}
-        user: ${env.POSTGRES_USER:=llamastack}
-        password: ${env.POSTGRES_PASSWORD:=llamastack}
-      responses_store:
-        type: sql_postgres
-        host: ${env.POSTGRES_HOST:=localhost}
-        port: ${env.POSTGRES_PORT:=5432}
-        db: ${env.POSTGRES_DB:=llamastack}
-        user: ${env.POSTGRES_USER:=llamastack}
-        password: ${env.POSTGRES_PASSWORD:=llamastack}
+      persistence:
+        agent_state:
+          namespace: agents
+          backend: kv_default
+        responses:
+          table_name: responses
+          backend: sql_default
+          max_write_queue_size: 10000
+          num_writers: 4
   post_training:
   - provider_id: huggingface-gpu
     provider_type: inline::huggingface-gpu
@@ -237,10 +232,10 @@ providers:
     config:
       kvstore:
         namespace: batches
-        backend: kv_postgres
+        backend: kv_default
 storage:
   backends:
-    kv_postgres:
+    kv_default:
       type: kv_postgres
       host: ${env.POSTGRES_HOST:=localhost}
       port: ${env.POSTGRES_PORT:=5432}
@@ -248,7 +243,7 @@ storage:
       user: ${env.POSTGRES_USER:=llamastack}
       password: ${env.POSTGRES_PASSWORD:=llamastack}
       table_name: ${env.POSTGRES_TABLE_NAME:=llamastack_kvstore}
-    sql_postgres:
+    sql_default:
       type: sql_postgres
       host: ${env.POSTGRES_HOST:=localhost}
       port: ${env.POSTGRES_PORT:=5432}
@@ -258,27 +253,44 @@ storage:
   stores:
     metadata:
       namespace: registry
-      backend: kv_postgres
+      backend: kv_default
     inference:
       table_name: inference_store
-      backend: sql_postgres
+      backend: sql_default
       max_write_queue_size: 10000
       num_writers: 4
     conversations:
       table_name: openai_conversations
-      backend: sql_postgres
+      backend: sql_default
     prompts:
       namespace: prompts
-      backend: kv_postgres
+      backend: kv_default
 registered_resources:
   models: []
-  shields: []
+  shields:
+  - shield_id: llama-guard
+    provider_id: ${env.SAFETY_MODEL:+llama-guard}
+    provider_shield_id: ${env.SAFETY_MODEL:=}
+  - shield_id: code-scanner
+    provider_id: ${env.CODE_SCANNER_MODEL:+code-scanner}
+    provider_shield_id: ${env.CODE_SCANNER_MODEL:=}
   vector_dbs: []
   datasets: []
   scoring_fns: []
   benchmarks: []
-  tool_groups: []
+  tool_groups:
+  - toolgroup_id: builtin::websearch
+    provider_id: tavily-search
+  - toolgroup_id: builtin::rag
+    provider_id: rag-runtime
 server:
   port: 8321
 telemetry:
   enabled: true
+vector_stores:
+  default_provider_id: faiss
+  default_embedding_model:
+    provider_id: sentence-transformers
+    model_id: nomic-ai/nomic-embed-text-v1.5
+safety:
+  default_shield_id: llama-guard