032: Phase 3 US1 — 13 mixins migrated to async + dashboard routes

T011-T017: All SupersetClient mixins now async.
T018: _detail_routes.py uses registry/AsyncSupersetClient.
T019: Partial — routes/environments, settings, profile, listing async.

RATIONALE: Big-bang merge of sync+AsyncSupersetClient.
REJECTED: dual-stack.

Remaining T019: assistant/*, migration, datasets, git helpers still use sync.

backend/src/api/routes/settings.py

@@ -22,7 +22,7 @@ from ...core.config_manager import ConfigManager
 from ...core.config_models import AppConfig, Environment, GlobalSettings, LoggingConfig
 from ...core.database import get_db
 from ...core.logger import belief_scope, logger
-from ...core.superset_client import SupersetClient
+from ...core.async_superset_client import AsyncSupersetClient
 from ...dependencies import get_config_manager, has_permission
 from ...models.config import AppConfigRecord
 from ...models.llm import ValidationPolicy
@@ -73,12 +73,12 @@ def _normalize_superset_env_url(raw_url: str) -> str:
 # @BRIEF Run lightweight Superset connectivity validation without full pagination scan.
 # @PRE  env contains valid URL and credentials.
 # @POST  Raises on auth/API failures; returns None on success.
-def _validate_superset_connection_fast(env: Environment) -> None:
-    client = SupersetClient(env)
+async def _validate_superset_connection_fast(env: Environment) -> None:
+    client = AsyncSupersetClient(env)
     # 1) Explicit auth check
-    client.authenticate()
+    await client.authenticate()
     # 2) Single lightweight API call to ensure read access
-    client.get_dashboards_page(
+    _, _ = await client.get_dashboards_page(
         query={
             "page": 0,
             "page_size": 1,
@@ -238,7 +238,7 @@ async def add_environment(
 
     # Validate connection before adding (fast path)
     try:
-        _validate_superset_connection_fast(env)
+        await _validate_superset_connection_fast(env)
     except Exception as e:
         logger.error(
             f"[add_environment][Coherence:Failed] Connection validation failed: {e}"
@@ -280,7 +280,7 @@ async def update_environment(
 
     # Validate connection before updating (fast path)
     try:
-        _validate_superset_connection_fast(env_to_validate)
+        await _validate_superset_connection_fast(env_to_validate)
     except Exception as e:
         logger.error(
             f"[update_environment][Coherence:Failed] Connection validation failed: {e}"
@@ -331,7 +331,7 @@ async def test_environment_connection(
         raise HTTPException(status_code=404, detail=f"Environment {id} not found")
 
     try:
-        _validate_superset_connection_fast(env)
+        await _validate_superset_connection_fast(env)
 
         logger.info(
             f"[test_environment_connection][Coherence:OK] Connection successful for {id}"