feat(suggest): GET /api/suggest, cacheable and DataFrame-free

A dedicated endpoint rather than a mode of /api/schools, because that
path filters and sorts 25,000 pandas rows per query while holding the
GIL — affordable once per search, not once per keystroke. A test asserts
the distinction directly by making load_school_data raise and requiring
the endpoint to answer anyway.

Nothing errors on ordinary input: a short query, no matches, or
Typesense being down are all 200 with an empty list.

Cached deliberately. Prefix queries repeat enormously across users and
school names change once a year, so s-maxage plus the existing ETag
middleware turns most keystrokes into 304s.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015mWQnpye9F299NVRCCSRvj

backend/app.py

@@ -33,6 +33,7 @@ from .data_loader import (
     get_supplementary_data,
     get_supplementary_data_batch,
     search_schools_typesense,
+    suggest_schools_typesense,
 )
 from .data_loader import get_data_info as get_db_info
 from . import flags
@@ -383,6 +384,7 @@ CACHE_RULES: list[tuple[str, tuple[int, int, int]]] = [
     ("/api/schools/",           (300,  3600,  86400)),  # /api/schools/{urn}
     ("/api/rankings",           (60,   600,   3600)),
     ("/api/compare",            (60,   600,   3600)),
+    ("/api/suggest",            (60,   3600,  86400)),  # autosuggest
     ("/api/schools",            (30,   300,   1800)),   # search list
 ]
 
@@ -1299,6 +1301,38 @@ async def get_place(request: Request, kind: str, slug: str,
     }
 
 
+# Two characters. One is not a query — it matches thousands of schools and the
+# response is useless, so it is not worth a round trip.
+SUGGEST_MIN_QUERY = 2
+
+
+@app.get("/api/suggest")
+@limiter.limit("120/minute")
+async def suggest_schools(
+    request: Request,
+    q: str = Query("", max_length=100),
+    limit: int = Query(8, ge=1, le=20),
+):
+    """School name suggestions, from Typesense alone.
+
+    Deliberately not a mode of /api/schools: that path filters and sorts the
+    full in-memory DataFrame, which is far too expensive to run per keystroke.
+
+    Nothing here returns an error for ordinary input. A short query, no
+    matches, or Typesense being unreachable are all 200 with an empty list —
+    a dropdown that quietly does not appear is the right failure for a
+    keystroke path, and there is no DataFrame fallback because the 25,000-row
+    substring scan is precisely what this endpoint exists to avoid.
+
+    120/minute rather than the default 60: a 200 ms debounce makes typing
+    legitimately bursty.
+    """
+    query = q.strip()
+    if len(query) < SUGGEST_MIN_QUERY:
+        return {"suggestions": []}
+    return {"suggestions": suggest_schools_typesense(query, limit)}
+
+
 @app.get("/api/flags")
 @limiter.limit(f"{settings.rate_limit_per_minute}/minute")
 async def get_feature_flags(request: Request):