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.

songs.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"context"
+	"database/sql"
 	"errors"
 	"fmt"
 	"log/slog"
@@ -9,8 +10,6 @@ import (
 	"os"
 	"strconv"
 	"time"
-
-	"github.com/jackc/pgx/v5"
 )
 
 const pageSize = 20
@@ -52,12 +51,12 @@ const songColumns = `
 	(select count(*) from reviews r where r.song_id = s.id),
 	case when s.submitted_by = $1
 	          or exists (select 1 from reviews r where r.song_id = s.id and r.reviewer_id = $1)
-	     then (select avg(r.score)::float from reviews r where r.song_id = s.id)
+	     then (select avg(r.score) from reviews r where r.song_id = s.id)
 	end,
 	s.submitted_by = $1,
 	exists (select 1 from reviews r where r.song_id = s.id and r.reviewer_id = $1)`
 
-func scanSongs(rows pgx.Rows) ([]*songSummary, error) {
+func scanSongs(rows *sql.Rows) ([]*songSummary, error) {
 	defer rows.Close()
 	var out []*songSummary
 	for rows.Next() {
@@ -74,7 +73,7 @@ func scanSongs(rows pgx.Rows) ([]*songSummary, error) {
 // The queue is a worklist: songs you can still review, oldest first, and never your own — you can
 // never act on those, so they would sit at the front forever.
 func (a *app) queue(ctx context.Context, viewerID, cursor int64) (*songList, error) {
-	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 <> $1
 		   and not exists (select 1 from reviews r where r.song_id = s.id and r.reviewer_id = $1)
@@ -93,7 +92,7 @@ func (a *app) queue(ctx context.Context, viewerID, cursor int64) (*songList, err
 
 // Everything, newest first. This is where a song lives once it has left the queue.
 func (a *app) browse(ctx context.Context, viewerID, cursor int64) (*songList, error) {
-	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 ($2 = 0 or s.id < $2)
 		 order by s.created_at desc, s.id desc
@@ -166,7 +165,7 @@ func (s *songDetail) LyricLines() []lyricLine { return parseLRC(s.Lyrics) }
 
 func (a *app) song(ctx context.Context, viewerID, songID int64) (*songDetail, error) {
 	var d songDetail
-	err := a.pool.QueryRow(ctx, `select`+songColumns+`,
+	err := a.db.QueryRowContext(ctx, `select`+songColumns+`,
 		       coalesce(s.description, ''), coalesce(s.lyrics, ''), s.source_url
 		  from songs s join users u on u.id = s.submitted_by
 		 where s.id = $2`, viewerID, songID).
@@ -207,13 +206,13 @@ func (a *app) song(ctx context.Context, viewerID, songID int64) (*songDetail, er
 // draining the queue never means navigating back to it.
 func (a *app) nextInQueue(ctx context.Context, viewerID, exceptID int64) (int64, error) {
 	var id int64
-	err := a.pool.QueryRow(ctx, `
+	err := a.db.QueryRowContext(ctx, `
 		select s.id from songs s
 		 where s.submitted_by <> $1 and s.id <> $2
 		   and not exists (select 1 from reviews r where r.song_id = s.id and r.reviewer_id = $1)
 		 order by s.created_at, s.id
 		 limit 1`, viewerID, exceptID).Scan(&id)
-	if errors.Is(err, pgx.ErrNoRows) {
+	if errors.Is(err, sql.ErrNoRows) {
 		return 0, nil
 	}
 	return id, err
@@ -226,7 +225,7 @@ func (a *app) songPage(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	d, err := a.song(r.Context(), memberFrom(r.Context()).ID, id)
-	if errors.Is(err, pgx.ErrNoRows) {
+	if errors.Is(err, sql.ErrNoRows) {
 		http.NotFound(w, r)
 		return
 	} else if err != nil {
@@ -246,7 +245,7 @@ func (a *app) editLyrics(w http.ResponseWriter, r *http.Request) {
 		http.NotFound(w, r)
 		return
 	}
-	tag, err := a.pool.Exec(r.Context(),
+	res, err := a.db.ExecContext(r.Context(),
 		`update songs set lyrics = nullif($3, '') where id = $1 and submitted_by = $2`,
 		id, memberFrom(r.Context()).ID, cleanLyrics(r.FormValue("lyrics")))
 	if err != nil {
@@ -254,7 +253,7 @@ func (a *app) editLyrics(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, "virhe", http.StatusInternalServerError)
 		return
 	}
-	if tag.RowsAffected() == 0 {
+	if affected(res) == 0 {
 		http.NotFound(w, r)
 		return
 	}
@@ -284,7 +283,7 @@ func (a *app) editSong(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	tag, err := a.pool.Exec(r.Context(), `
+	res, err := a.db.ExecContext(r.Context(), `
 		update songs set title = $3, artist = $4, genre = $5, description = nullif($6, '')
 		 where id = $1 and submitted_by = $2
 		   and not exists (select 1 from reviews r where r.song_id = songs.id)`,
@@ -295,7 +294,7 @@ func (a *app) editSong(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, "virhe", http.StatusInternalServerError)
 		return
 	}
-	if tag.RowsAffected() == 0 {
+	if affected(res) == 0 {
 		a.flash(w, "Kappaletta ei voi enää muokata — sitä on jo arvosteltu.")
 	} else {
 		a.flash(w, "Tiedot tallennettu.")
@@ -310,7 +309,7 @@ func (a *app) deleteSong(w http.ResponseWriter, r *http.Request) {
 		http.NotFound(w, r)
 		return
 	}
-	tag, err := a.pool.Exec(r.Context(), `
+	res, err := a.db.ExecContext(r.Context(), `
 		delete from songs where id = $1 and submitted_by = $2
 		  and not exists (select 1 from reviews r where r.song_id = songs.id)`,
 		id, memberFrom(r.Context()).ID)
@@ -319,7 +318,7 @@ func (a *app) deleteSong(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, "virhe", http.StatusInternalServerError)
 		return
 	}
-	if tag.RowsAffected() == 0 {
+	if affected(res) == 0 {
 		a.flash(w, "Kappaletta ei voi enää poistaa — sitä on jo arvosteltu.")
 		http.Redirect(w, r, fmt.Sprintf("/songs/%d", id), http.StatusSeeOther)
 		return
@@ -348,8 +347,8 @@ func (a *app) audio(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	var name string
-	err = a.pool.QueryRow(r.Context(), `select audio_file from songs where id = $1`, id).Scan(&name)
-	if errors.Is(err, pgx.ErrNoRows) {
+	err = a.db.QueryRowContext(r.Context(), `select audio_file from songs where id = $1`, id).Scan(&name)
+	if errors.Is(err, sql.ErrNoRows) {
 		http.NotFound(w, r)
 		return
 	} else if err != nil {