Rewrite Python comments in Google developer documentation style (#12)

* Rewrite comments in Google developer documentation style

Rewrite the comments and docstrings across the backend core modules so they
read plainly. The previous prose was accurate but dense and figurative, which
made it slow to skim.

Applies the Google developer documentation style guide: short sentences, active
voice, present tense, American spelling, and no metaphors, idioms, or
rhetorical asides. Replaces em-dash chains with separate sentences.

backend/tests/test_embedding_model_switch.py

@@ -1,21 +1,21 @@
 """Switching embedding models must re-embed the bank.
 
-Vectors from two different models are not comparable — different space, often
-different width — so changing the model has to throw the stored ones away and
-let the post-turn pass rebuild them.
+Vectors from two different models are not comparable. They live in
+different spaces and often have different widths. Changing the model must
+discard the stored vectors and let the post-turn pass rebuild them.
 
-That worked while the vectors lived in `memories.embedding`: the settings
-route nulled that column and the embed queue picked the rows up. Migration 38
-moved the vectors to `embedding_blob` with an `embedded` flag beside them, and
-the bulk clear kept nulling the old column alone. The blob survived, the flag
-stayed true, `_embed_pending` (which looks for `embedded IS FALSE`) never saw
-the rows, and the bank went on ranking against the previous model's vectors
-for good.
+This worked while the vectors lived in `memories.embedding`. The settings
+route nulled that column, and the embed queue picked up the rows. Migration
+38 moved the vectors to `embedding_blob` and added an `embedded` flag beside
+them, but the bulk clear kept nulling only the old column. The blob
+survived, the flag stayed true, and `_embed_pending` (which filters on
+`embedded IS FALSE`) never saw the rows. The bank kept ranking against the
+previous model's vectors.
 
-Nothing reports this. `cosine` returns 0.0 on a width mismatch, so a
-different-width model scores every memory zero and retrieval quietly returns
-whichever rows sort first; a same-width model scores plausible-looking
-garbage.
+Nothing reports this failure. `cosine` returns 0.0 on a width mismatch, so a
+different-width model scores every memory zero, and retrieval silently
+returns whichever rows sort first. A same-width model scores plausible
+garbage instead.
 
     python -m pytest tests/test_embedding_model_switch.py -v
 """
@@ -114,8 +114,9 @@ def test_changing_the_model_clears_every_vector(client):
 
 
 def test_cleared_memories_are_queued_for_re_embedding(client):
-    """The flag is not cosmetic: it is the only thing `_embed_pending` filters
-    on, so this is the assertion that the bank actually recovers."""
+    """The `embedded` flag is not cosmetic. It is the only condition
+    `_embed_pending` filters on, so this test confirms the bank actually
+    recovers."""
     client.put("/api/settings", json={"embedding_model": "model-b"})
 
     db = SessionLocal()
@@ -155,7 +156,7 @@ def test_retrieval_uses_no_stale_vector_after_the_switch(client, monkeypatch):
 
 
 def test_an_unrelated_settings_change_keeps_the_vectors(client):
-    """Only an embedding-model change may clear the bank — re-embedding costs
+    """Only an embedding-model change may clear the bank. Re-embedding costs
     an API call per memory."""
     r = client.put("/api/settings", json={"model": "some-other-chat-model"})
     assert r.status_code == 200, r.text