refactor: rename similar → nearby, so the code says what the section does

The section ranks on distance and is headed "Other schools nearby", but every
identifier still called it "similar" — the exact drift that leaves a later
reader trusting a name over the behaviour.

Mechanical: files, the module, the payload key, the type, the components, the
prop. No behaviour change; the suites are unchanged in count and still green.
Free to do now because #150 has not merged, so the payload key rename needs no
lockstep deploy. Uses of "similar" that are ordinary English — progress
measures compared to similar pupils, and unrelated comments — are untouched.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/schemas.py

@@ -536,7 +536,7 @@ RANKING_COLUMNS = [
 # include. All-through schools appear in both primary and secondary results,
 # which is why this is a set per phase rather than a single string comparison.
 #
-# Lives here rather than in app.py because similar_schools.py needs it too, and
+# Lives here rather than in app.py because nearby_schools.py needs it too, and
 # importing app from there would be a cycle.
 PHASE_GROUPS: dict[str, set[str]] = {
     "primary": {"primary", "middle deemed primary", "all-through"},