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.

stats.go

@@ -55,13 +55,18 @@ type stats struct {
 	MostProlific []userStat
 }
 
+// SQLite has no stddev aggregate. This is the population formula written out; max() absorbs the
+// tiny negative that floating-point cancellation produces when every score is identical, which
+// would otherwise make sqrt() return null and fail the scan.
+const stddevPop = `sqrt(max(0.0, avg(r.score * r.score) - avg(r.score) * avg(r.score)))`
+
 // Every leaderboard is ordered and limited in SQL, and every one carries a deterministic tie-break:
-// ties are common in a ten-person club, and without one Postgres may return a different ten each
-// time, so the page visibly reshuffles between reloads for no reason.
+// ties are common in a ten-person club, and without one the database may return a different ten
+// each time, so the page visibly reshuffles between reloads for no reason.
 func (a *app) songLeaderboard(ctx context.Context, valueExpr, direction string) ([]songStat, error) {
-	rows, err := a.pool.Query(ctx, `
-		select s.id, s.title, s.artist, `+valueExpr+`::float as value, count(r.id)::int as reviews,
-		       min(r.score)::int, max(r.score)::int
+	rows, err := a.db.QueryContext(ctx, `
+		select s.id, s.title, s.artist, cast(`+valueExpr+` as real) as value,
+		       count(r.id) as reviews, min(r.score), max(r.score)
 		  from songs s join reviews r on r.song_id = s.id
 		 group by s.id
 		having count(r.id) >= $1
@@ -86,8 +91,8 @@ func (a *app) songLeaderboard(ctx context.Context, valueExpr, direction string)
 // Reviewer boards need a minimum too, or one enthusiastic 100 makes someone the most generous
 // member in the club forever.
 func (a *app) reviewerLeaderboard(ctx context.Context, valueExpr, direction string) ([]userStat, error) {
-	rows, err := a.pool.Query(ctx, `
-		select u.id, u.name, u.avatar, `+valueExpr+`::float as value, count(r.id)::int as n
+	rows, err := a.db.QueryContext(ctx, `
+		select u.id, u.name, u.avatar, cast(`+valueExpr+` as real) as value, count(r.id) as n
 		  from users u join reviews r on r.reviewer_id = u.id
 		 group by u.id
 		having count(r.id) >= $1
@@ -109,8 +114,8 @@ func (a *app) reviewerLeaderboard(ctx context.Context, valueExpr, direction stri
 }
 
 func (a *app) mostProlific(ctx context.Context) ([]userStat, error) {
-	rows, err := a.pool.Query(ctx, `
-		select u.id, u.name, u.avatar, count(s.id)::float, count(s.id)::int
+	rows, err := a.db.QueryContext(ctx, `
+		select u.id, u.name, u.avatar, cast(count(s.id) as real), count(s.id)
 		  from users u join songs s on s.submitted_by = u.id
 		 group by u.id
 		 order by count(s.id) desc, u.id asc
@@ -140,11 +145,8 @@ func (a *app) statsPage(w http.ResponseWriter, r *http.Request) {
 	for _, load := range []func() error{
 		func() (err error) { s.TopSongs, err = a.songLeaderboard(ctx, "avg(r.score)", "desc"); return },
 		func() (err error) { s.BottomSongs, err = a.songLeaderboard(ctx, "avg(r.score)", "asc"); return },
-		func() (err error) {
-			s.MostDivisive, err = a.songLeaderboard(ctx, "stddev_pop(r.score)", "desc")
-			return
-		},
-		func() (err error) { s.MostUnified, err = a.songLeaderboard(ctx, "stddev_pop(r.score)", "asc"); return },
+		func() (err error) { s.MostDivisive, err = a.songLeaderboard(ctx, stddevPop, "desc"); return },
+		func() (err error) { s.MostUnified, err = a.songLeaderboard(ctx, stddevPop, "asc"); return },
 		func() (err error) { s.MostReviewed, err = a.songLeaderboard(ctx, "count(r.id)", "desc"); return },
 		func() (err error) { s.Harshest, err = a.reviewerLeaderboard(ctx, "avg(r.score)", "asc"); return },
 		func() (err error) { s.MostGenerous, err = a.reviewerLeaderboard(ctx, "avg(r.score)", "desc"); return },