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.

reviews.go

@@ -2,14 +2,13 @@ package main
 
 import (
 	"context"
+	"database/sql"
 	"errors"
 	"fmt"
 	"log/slog"
 	"net/http"
 	"strconv"
 	"time"
-
-	"github.com/jackc/pgx/v5"
 )
 
 const (
@@ -17,6 +16,10 @@ const (
 	maxReview  = 5000
 )
 
+// The same window as a SQLite date modifier, for the two statements that enforce it. SQLite has no
+// interval type to bind, so the unit travels in the string.
+var editWindowAgo = fmt.Sprintf("-%d seconds", int(editWindow.Seconds()))
+
 type review struct {
 	ID         int64
 	SongID     int64
@@ -40,7 +43,7 @@ func (r *review) Initials() string {
 }
 
 func (a *app) reviewsFor(ctx context.Context, songID, viewerID int64) ([]*review, error) {
-	rows, err := a.pool.Query(ctx, `
+	rows, err := a.db.QueryContext(ctx, `
 		select r.id, r.song_id, r.reviewer_id, u.name, r.score, r.text, r.created_at, r.updated_at,
 		       r.reviewer_id = $2
 		  from reviews r join users u on u.id = r.reviewer_id
@@ -64,13 +67,13 @@ func (a *app) reviewsFor(ctx context.Context, songID, viewerID int64) ([]*review
 
 func (a *app) viewerReview(ctx context.Context, songID, viewerID int64) (*review, error) {
 	var v review
-	err := a.pool.QueryRow(ctx, `
+	err := a.db.QueryRowContext(ctx, `
 		select r.id, r.song_id, r.reviewer_id, u.name, r.score, r.text, r.created_at, r.updated_at, true
 		  from reviews r join users u on u.id = r.reviewer_id
 		 where r.song_id = $1 and r.reviewer_id = $2`, songID, viewerID).
 		Scan(&v.ID, &v.SongID, &v.ReviewerID, &v.Reviewer, &v.Score, &v.Text,
 			&v.CreatedAt, &v.UpdatedAt, &v.Own)
-	if errors.Is(err, pgx.ErrNoRows) {
+	if errors.Is(err, sql.ErrNoRows) {
 		return nil, nil
 	}
 	return &v, err
@@ -105,8 +108,8 @@ func (a *app) createReview(w http.ResponseWriter, r *http.Request) {
 	// You cannot review your own song, and the unique constraint is what stops a second review —
 	// no read-then-write race to lose.
 	var submitter int64
-	err = a.pool.QueryRow(r.Context(), `select submitted_by from songs where id = $1`, songID).Scan(&submitter)
-	if errors.Is(err, pgx.ErrNoRows) {
+	err = a.db.QueryRowContext(r.Context(), `select submitted_by from songs where id = $1`, songID).Scan(&submitter)
+	if errors.Is(err, sql.ErrNoRows) {
 		http.NotFound(w, r)
 		return
 	} else if err != nil {
@@ -119,7 +122,7 @@ func (a *app) createReview(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	_, err = a.pool.Exec(r.Context(),
+	_, err = a.db.ExecContext(r.Context(),
 		`insert into reviews (song_id, reviewer_id, score, text) values ($1, $2, $3, $4)`,
 		songID, me.ID, score, text)
 	if isUnique(err) {
@@ -153,12 +156,12 @@ func (a *app) editReview(w http.ResponseWriter, r *http.Request) {
 	}
 
 	var songID int64
-	err = a.pool.QueryRow(r.Context(), `
-		update reviews set score = $3, text = $4, updated_at = now()
-		 where id = $1 and reviewer_id = $2 and updated_at > now() - $5::interval
+	err = a.db.QueryRowContext(r.Context(), `
+		update reviews set score = $3, text = $4, updated_at = datetime('now')
+		 where id = $1 and reviewer_id = $2 and updated_at > datetime('now', $5)
 		 returning song_id`,
-		id, memberFrom(r.Context()).ID, score, text, editWindow.String()).Scan(&songID)
-	if errors.Is(err, pgx.ErrNoRows) {
+		id, memberFrom(r.Context()).ID, score, text, editWindowAgo).Scan(&songID)
+	if errors.Is(err, sql.ErrNoRows) {
 		a.flash(w, "Muokkausaika on umpeutunut.")
 		http.Redirect(w, r, r.FormValue("from"), http.StatusSeeOther)
 		return
@@ -180,12 +183,12 @@ func (a *app) deleteReview(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	var songID int64
-	err = a.pool.QueryRow(r.Context(), `
+	err = a.db.QueryRowContext(r.Context(), `
 		delete from reviews
-		 where id = $1 and reviewer_id = $2 and updated_at > now() - $3::interval
+		 where id = $1 and reviewer_id = $2 and updated_at > datetime('now', $3)
 		 returning song_id`,
-		id, memberFrom(r.Context()).ID, editWindow.String()).Scan(&songID)
-	if errors.Is(err, pgx.ErrNoRows) {
+		id, memberFrom(r.Context()).ID, editWindowAgo).Scan(&songID)
+	if errors.Is(err, sql.ErrNoRows) {
 		a.flash(w, "Muokkausaika on umpeutunut.")
 		http.Redirect(w, r, r.FormValue("from"), http.StatusSeeOther)
 		return