feat(CRIT-010): add oil_prices API endpoint with zone-to-county mapping

Add GET /oil-prices/county/:county_id public endpoint to serve scraped
oil price data by county. Includes oil_prices table definition in schema
and a backfill script to populate county_id on existing records.

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

schema.sql

@@ -58,6 +58,22 @@ CREATE TABLE listings (
     last_edited TIMESTAMPTZ DEFAULT CURRENT_TIMESTAMP
 );
 
+-- Oil prices (scraped from NewEnglandOil.com / MaineOil.com)
+CREATE TABLE oil_prices (
+    id SERIAL PRIMARY KEY,
+    state VARCHAR(100),
+    zone INTEGER,
+    name VARCHAR(255),
+    price DOUBLE PRECISION,
+    date VARCHAR(20),
+    scrapetimestamp TIMESTAMPTZ DEFAULT CURRENT_TIMESTAMP,
+    company_id INTEGER,
+    county_id INTEGER
+);
+
+-- If the table already exists, add county_id
+-- ALTER TABLE oil_prices ADD COLUMN county_id INTEGER;
+
 -- If the table already exists, add the new columns
 -- ALTER TABLE listings ADD COLUMN price_per_gallon_cash DOUBLE PRECISION;
 -- ALTER TABLE listings ADD COLUMN note TEXT;