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.

Dockerfile

@@ -10,7 +10,9 @@ RUN CGO_ENABLED=0 go build -ldflags "-X main.version=${VERSION}" -o /levyraati .
 FROM alpine:3.24
 # yt-dlp rots against YouTube. Alpine's active branch tracks it closely (3.24 carries the current
 # release), so a rebuild is the update — and this avoids python3 + pip in the image entirely.
-RUN apk add --no-cache ffmpeg yt-dlp ca-certificates
+# sqlite is the CLI only — the app links its own pure-Go copy. It is here so `.backup` and a shell
+# are reachable with docker compose exec, which is the whole of database operations now.
+RUN apk add --no-cache ffmpeg yt-dlp ca-certificates sqlite
 COPY --from=build /levyraati /usr/local/bin/levyraati
 ENV STORAGE_DIR=/storage
 EXPOSE 8080