feat: add admin panel, stats endpoint, and url field to listings

- Add full admin CRUD routes for users, companies, listings, oil-prices
  protected behind auth middleware (/admin/*)
- Add public /stats endpoint returning latest market price aggregates
- Add /health and / health check endpoints
- Add `url` field to listings (struct, all SQL queries, create/update)
- Add `phone` and `url` fields to OilPrice struct
- Remove deprecated company CRUD handlers (get_company, create_company)
- Update schema.sql; remove one-off alter/drop migration scripts

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

schema.sql

@@ -53,6 +53,7 @@ CREATE TABLE listings (
     online_ordering VARCHAR(20) NOT NULL DEFAULT 'none',
     county_id INTEGER NOT NULL,
     town VARCHAR(100),
+    url VARCHAR(255),
     user_id INTEGER NOT NULL,
     created_at TIMESTAMPTZ DEFAULT CURRENT_TIMESTAMP,
     last_edited TIMESTAMPTZ DEFAULT CURRENT_TIMESTAMP
@@ -82,3 +83,10 @@ CREATE TABLE oil_prices (
 
 -- If the table already exists, drop the company_id column
 -- ALTER TABLE listings DROP COLUMN company_id;
+
+CREATE TABLE stats_prices (
+    id SERIAL PRIMARY KEY,
+    state VARCHAR(2) NOT NULL,
+    price DOUBLE PRECISION NOT NULL,
+    created_at TIMESTAMPTZ DEFAULT CURRENT_TIMESTAMP
+);