fix(search): the live-search tooltip says what a search costs

It always said a live search spends the shared API quota, while the default source (the website's
own endpoint, scrape) spends none — the owner avoided a free search because of it. /api/me now names
the running search_source, and the tooltip names the cost under it: none for scrape (the enrich step
costs a unit or two), 100 units a page for the official API.

backend/tests/test_me_search_source.py

@@ -0,0 +1,26 @@
+"""`/api/me` names the live-search source, so the search button's tooltip can say what a search costs
+under the source that actually runs (the default website source spends no API quota)."""
+
+from app import sysconfig
+
+_PW = "correct horse battery"
+
+
+def _me(client):
+    r = client.get("/api/me")
+    assert r.status_code == 200 and r.json()["authenticated"] is True
+    return r.json()
+
+
+def test_me_names_the_running_search_source(client, db, make_user):
+    # /api/me resolves its user through optional_current_user, which the as_user override does not
+    # cover — so sign in for real (see test_me_email_flag).
+    user = make_user(password=_PW)
+    login = client.post("/auth/password-login", json={"email": user.email, "password": _PW})
+    assert login.status_code == 200
+
+    assert _me(client)["search_source"] == "scrape"  # the default
+
+    sysconfig.set_value(db, "search_source", "api")
+    db.commit()
+    assert _me(client)["search_source"] == "api"