Replace Postgres with SQLite

Ten members and a handful of songs a week never needed a database server,
and the server was the last thing making this a two-container deployment.
modernc.org/sqlite is pure Go, so CGO_ENABLED=0 survives and the dependency
count is unchanged: pgx out, sqlite in.

The port stayed small because the driver matches $1-style placeholders
against argument ordinals exactly as pgx does, so no query needed rewriting
for parameters. What did change:

- timestamptz becomes timestamp holding UTC 'YYYY-MM-DD HH:MM:SS'. The
  declared type is what makes the driver return time.Time, and the
  fixed-width UTC string is what makes ordering and comparison against
  datetime('now') mean what they say.
- interval has no equivalent: sessions.idle_ttl is seconds, and the review
  edit window travels as a SQLite date modifier string.
- No stddev_pop, so the divisive and unified boards spell the population
  formula out, guarded with max(0.0, ...) because cancellation returns a
  tiny negative when every score is identical.
- foreign_keys is off by default, so the cascades only exist because the
  pragma is set on every connection.

Drops the postgres service, its healthcheck, the depends_on gate, the
startup retry loop and POSTGRES_PASSWORD. ./storage is now the whole
backup. Tests get a fresh database file per test and run everywhere
instead of skipping without TEST_DATABASE_URL.

docker-compose.yml

@@ -1,28 +1,10 @@
 services:
-  postgres:
-    image: postgres:18-alpine
-    environment:
-      POSTGRES_USER: levyraati
-      POSTGRES_PASSWORD: ${POSTGRES_PASSWORD:?set POSTGRES_PASSWORD in .env}
-      POSTGRES_DB: levyraati
-    volumes:
-      # Postgres 18 keeps its data in /var/lib/postgresql/<version>/docker, so the mount is the
-      # parent directory, not the old /var/lib/postgresql/data.
-      - ./pgdata:/var/lib/postgresql
-    healthcheck:
-      test: ["CMD-SHELL", "pg_isready -U levyraati"]
-      interval: 5s
-      timeout: 3s
-      retries: 10
-    restart: unless-stopped
-
   app:
     build:
       context: .
       args:
         VERSION: ${VERSION:-dev}
     environment:
-      DATABASE_URL: postgres://levyraati:${POSTGRES_PASSWORD}@postgres:5432/levyraati
       ADMIN_USER: ${ADMIN_USER:-admin}
       ADMIN_PASSWORD: ${ADMIN_PASSWORD:?set ADMIN_PASSWORD in .env}
       ADDR: ":8080"
@@ -31,12 +13,10 @@ services:
       ADMIN_ADDR: ":8081"
       SECURE_COOKIES: ${SECURE_COOKIES:-true}
       PUBLIC_URL: ${PUBLIC_URL:-}
+    # The SQLite file sits in here beside the audio, so this one mount is the whole backup.
     volumes:
       - ./storage:/storage
     ports:
       - "8080:8080"
       - "8081:8081"
-    depends_on:
-      postgres:
-        condition: service_healthy
     restart: unless-stopped