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.

profile.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"context"
+	"database/sql"
 	"errors"
 	"io"
 	"log/slog"
@@ -12,7 +13,6 @@ import (
 	"strings"
 	"time"
 
-	"github.com/jackc/pgx/v5"
 	"golang.org/x/crypto/bcrypt"
 )
 
@@ -47,12 +47,12 @@ func (a *app) avatarPath(userID int64) string {
 // per-song opinion is gated, whole-history aggregate is public.
 func (a *app) profile(ctx context.Context, viewerID, userID int64) (*profileView, error) {
 	var p profileView
-	err := a.pool.QueryRow(ctx, `
+	err := a.db.QueryRowContext(ctx, `
 		select u.id, u.name, u.email, u.avatar, u.created_at,
 		       (select count(*) from songs s where s.submitted_by = u.id),
 		       (select count(*) from reviews r where r.reviewer_id = u.id),
-		       (select avg(r.score)::float from reviews r where r.reviewer_id = u.id),
-		       (select avg(r.score)::float from reviews r
+		       (select avg(r.score) from reviews r where r.reviewer_id = u.id),
+		       (select avg(r.score) from reviews r
 		          join songs s on s.id = r.song_id where s.submitted_by = u.id)
 		  from users u where u.id = $1`, userID).
 		Scan(&p.ID, &p.Name, &p.Email, &p.Avatar, &p.CreatedAt,
@@ -67,7 +67,7 @@ func (a *app) profile(ctx context.Context, viewerID, userID int64) (*profileView
 	}
 
 	// Their songs, with the viewer's own reveal rule applied to each average.
-	rows, err := a.pool.Query(ctx, `select`+songColumns+`
+	rows, err := a.db.QueryContext(ctx, `select`+songColumns+`
 		  from songs s join users u on u.id = s.submitted_by
 		 where s.submitted_by = $2
 		 order by s.created_at desc`, viewerID, userID)
@@ -90,7 +90,7 @@ func (a *app) profilePage(w http.ResponseWriter, r *http.Request) {
 		id = parsed
 	}
 	p, err := a.profile(r.Context(), me.ID, id)
-	if errors.Is(err, pgx.ErrNoRows) {
+	if errors.Is(err, sql.ErrNoRows) {
 		http.NotFound(w, r)
 		return
 	} else if err != nil {
@@ -120,7 +120,7 @@ func (a *app) editProfile(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	if _, err := a.pool.Exec(r.Context(),
+	if _, err := a.db.ExecContext(r.Context(),
 		`update users set name = $2, email = $3 where id = $1`, me.ID, name, email); isUnique(err) {
 		a.flash(w, "Sähköpostiosoite on jo käytössä.")
 		http.Redirect(w, r, "/profile", http.StatusSeeOther)
@@ -153,7 +153,7 @@ func (a *app) editProfile(w http.ResponseWriter, r *http.Request) {
 
 func (a *app) changePassword(w http.ResponseWriter, r *http.Request, userID int64, current, next string) bool {
 	var hash string
-	if err := a.pool.QueryRow(r.Context(),
+	if err := a.db.QueryRowContext(r.Context(),
 		`select password_hash from users where id = $1`, userID).Scan(&hash); err != nil {
 		http.Error(w, "virhe", http.StatusInternalServerError)
 		return false
@@ -168,13 +168,13 @@ func (a *app) changePassword(w http.ResponseWriter, r *http.Request, userID int6
 		http.Error(w, "virhe", http.StatusInternalServerError)
 		return false
 	}
-	if _, err := a.pool.Exec(r.Context(),
+	if _, err := a.db.ExecContext(r.Context(),
 		`update users set password_hash = $2 where id = $1`, userID, string(newHash)); err != nil {
 		http.Error(w, "virhe", http.StatusInternalServerError)
 		return false
 	}
 	// Every other session dies; this browser keeps its own.
-	if _, err := a.pool.Exec(r.Context(),
+	if _, err := a.db.ExecContext(r.Context(),
 		`delete from sessions where user_id = $1 and token <> $2`, userID, sessionToken(r)); err != nil {
 		slog.Error("drop sessions", "ctx", "auth", "error", err, "user", userID)
 	}
@@ -202,7 +202,7 @@ func (a *app) saveAvatar(r *http.Request, userID int64, file io.Reader) error {
 	if err := toAvatarJPEG(r.Context(), tmp, out); err != nil {
 		return err
 	}
-	_, err = a.pool.Exec(r.Context(),
+	_, err = a.db.ExecContext(r.Context(),
 		`update users set avatar = $2 where id = $1`, userID, filepath.Base(out))
 	return err
 }