fix(admissions): correct first_preference_offer_pct in dbt staging

The staging model was mapping EES column ``proportion_1stprefs_v_totaloffers``
straight onto ``first_preference_offer_pct``. That raw column is not a
percentage — it is a ratio of first-preference applications to total offers
(an oversubscription indicator, >1 means oversubscribed), so OLQH rendered
as "1%" when the true first-choice success rate is 27/42 = 64%.

The frontend display code is not at fault and is not patched here —
data-quality issues must be fixed at the source.

- stg_ees_admissions: compute ``first_preference_offer_pct`` as
  ``100 * number_1st_preference_offers / times_put_as_1st_preference`` —
  of families who listed this school first, the % that received an offer
  (0–100). Guard against divide-by-zero.
- stg_ees_admissions: expose the legitimate EES ratio as the new column
  ``oversubscription_ratio`` (1st-preference applications per place) for
  future use, clearly named.
- fact_admissions, FactAdmissions model, data_loader: propagate the new
  ``oversubscription_ratio`` column.
- SchoolAdmissions type: document both columns inline.
- buildSchoolSummary: reword the oversubscription clause so it reads
  sensibly across the whole 0–100 range (no more "just 64%").
- Hero chip subtitle: clearer phrasing "X% of first-choice applicants
  offered a place".

Requires a dbt run of stg_ees_admissions and fact_admissions on deploy
so the new column materialises.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/models.py

@@ -184,6 +184,7 @@ class FactAdmissions(Base):
     first_preference_applications = Column(Integer)
     first_preference_offers = Column(Integer)
     first_preference_offer_pct = Column(Float)
+    oversubscription_ratio = Column(Float)
     oversubscribed = Column(Boolean)
     admissions_policy = Column(String(100))