refactor: read settings directly instead of getattr with a second default

About 85 call sites read a setting as getattr(settings, "NAME", fallback),
each carrying its own copy of the default. Every one of those names is a
field with a default on the model, so the fallback could never apply to
the real settings object; it only masked drift. Two had drifted:

- OPENAI_PROMPT_CACHE_KEY defaults to True on the model but the reader
  fell back to False, and two test stubs relied on that.
- SharePoint's MICROSOFT_AUTHORITY fallback to
  https://login.microsoftonline.com/<tenant> never fired, because the
  attribute always exists (as None), so MSAL got authority=None. The
  connector now derives the tenant authority when the setting is unset,
  as its test always assumed.

Four places read EMBEDDINGS_KEY straight from os.environ, skipping the
"None"/"" normalisation the model applies; they read the setting now.
Test stubs that replaced a module's settings with a SimpleNamespace list
every setting the code under test reads.

docsgpt/storage/db/bootstrap.py

@@ -96,7 +96,7 @@ def _release_boot_only_embeddings(log: logging.Logger) -> None:
 
     if settings.EMBEDDINGS_BASE_URL:
         return
-    if getattr(settings, "EMBEDDINGS_DELEGATE_TO_WORKER", False) is not True:
+    if settings.EMBEDDINGS_DELEGATE_TO_WORKER is not True:
         return
 
     import gc
@@ -144,8 +144,8 @@ def ensure_vector_schema(*, logger: Optional[logging.Logger] = None) -> None:
         )
         return
 
-    dsn = getattr(settings, "PGVECTOR_CONNECTION_STRING", None)
-    if not dsn and getattr(settings, "POSTGRES_URI", None):
+    dsn = settings.PGVECTOR_CONNECTION_STRING
+    if not dsn and settings.POSTGRES_URI:
         from docsgpt.core.db_uri import normalize_pgvector_connection_string
 
         dsn = normalize_pgvector_connection_string(settings.POSTGRES_URI)
@@ -172,7 +172,7 @@ def ensure_vector_schema(*, logger: Optional[logging.Logger] = None) -> None:
 
     dim: Optional[int] = dimension_for(settings.EMBEDDINGS_NAME)
 
-    graph_enabled = bool(getattr(settings, "GRAPHRAG_ENABLED", False))
+    graph_enabled = bool(settings.GRAPHRAG_ENABLED)
     started = time.monotonic()
     # A plain connection, never the store's pool: this can run pre-fork under
     # ``gunicorn --preload``, and an inherited pooled socket is a broken one.