Refuse invalid settings at start, health check follows PORT (closes #31)

PORT (1 to 65535) and XDG_DATA_HOME (an absolute path) are now checked in
the config package, which is built before the database opens, so a bad
value stops the start with an error and a non-zero exit instead of leaving
the daemon running without HTTP or putting the database somewhere else.
entrypoint.sh refuses a MALLOC_ARENA_MAX that is not a positive whole
number, since glibc ignores a bad one silently. The HEALTHCHECK probes the
port PORT names. The README corrects the Linux state directory, lists
XDG_DATA_HOME, and adds a "Running under upaas" section.

Model: opus-5-5

internal/server/handlers_test.go

@@ -8,6 +8,7 @@ import (
 	"testing"
 	"time"
 
+	"git.eeqj.de/sneak/routewatch/internal/config"
 	"git.eeqj.de/sneak/routewatch/internal/database"
 	"git.eeqj.de/sneak/routewatch/internal/logger"
 	"git.eeqj.de/sneak/routewatch/internal/metrics"
@@ -38,7 +39,7 @@ func (d blockingStatsDB) GetStatsContext(_ context.Context) (database.Stats, err
 func TestStatsHandlersDoNotLeakOnTimeout(t *testing.T) {
 	release := make(chan struct{})
 	db := blockingStatsDB{release: release}
-	s := New(db, streamer.New(logger.New(), metrics.New()), logger.New())
+	s := New(db, streamer.New(logger.New(), metrics.New()), logger.New(), &config.Config{})
 
 	handlers := map[string]http.HandlerFunc{
 		"status.json": s.handleStatusJSON(),