Queue SQLite writes on one connection so none fails as locked (closes #223)

internal/database now opens the database with one connection. pixa's
own reads and writes wait for it in turn instead of competing for
SQLite's lock, where a write that kept losing could wait past the
five-second busy timeout and fail with "database is locked". The busy
timeout stays, for another program writing to the same file.

With one connection, a query run while rows or a transaction are still
open would wait forever. No code in internal/database or
internal/imgcache does that; the comment on DB() tells callers.
README.md says pixa uses one connection.

Model: opus-5-5

internal/database/database.go

@@ -237,17 +237,18 @@ func ApplyMigrations(ctx context.Context, db *sql.DB, log *slog.Logger) error {
 	return nil
 }
 
-// DB returns the underlying sql.DB.
+// DB returns the underlying sql.DB. It has one connection, so close any
+// rows and end any transaction before running another query on it; a
+// query run while they are open waits forever.
 func (s *Database) DB() *sql.DB {
 	return s.db
 }
 
 func (s *Database) connect(ctx context.Context) error {
-	// Requests and the eviction pass write on separate connections. With
-	// a busy timeout, a write that finds another one in progress waits up
-	// to five seconds for it instead of failing at once with "database is
-	// locked". The driver runs each _pragma parameter on every connection
-	// it opens.
+	// With a busy timeout, a write that finds another program writing to
+	// the same database file waits up to five seconds for it instead of
+	// failing at once with "database is locked". The driver runs each
+	// _pragma parameter on every connection it opens.
 	separator := "?"
 	if strings.Contains(s.config.DBURL, "?") {
 		separator = "&"
@@ -264,6 +265,11 @@ func (s *Database) connect(ctx context.Context) error {
 		return err
 	}
 
+	// One connection: pixa's own reads and writes wait for it in turn
+	// instead of competing for SQLite's lock, where a write that keeps
+	// losing can wait past the busy timeout and be lost.
+	db.SetMaxOpenConns(1)
+
 	err = db.PingContext(ctx)
 	if err != nil {
 		s.log.Error("failed to ping database", "error", err)