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/server.go

@@ -4,9 +4,10 @@ package server
 import (
 	"context"
 	"net/http"
-	"os"
+	"strconv"
 	"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/streamer"
@@ -33,16 +34,18 @@ type Server struct {
 	db         database.Store
 	streamer   *streamer.Streamer
 	logger     *logger.Logger
+	port       int
 	srv        *http.Server
 	asnFetcher ASNFetcher
 }
 
 // New creates a new HTTP server
-func New(db database.Store, streamer *streamer.Streamer, logger *logger.Logger) *Server {
+func New(db database.Store, streamer *streamer.Streamer, logger *logger.Logger, cfg *config.Config) *Server {
 	s := &Server{
 		db:       db,
 		streamer: streamer,
 		logger:   logger,
+		port:     cfg.Port,
 	}
 
 	s.setupRoutes()
@@ -52,11 +55,6 @@ func New(db database.Store, streamer *streamer.Streamer, logger *logger.Logger)
 
 // Start starts the HTTP server
 func (s *Server) Start() error {
-	port := os.Getenv("PORT")
-	if port == "" {
-		port = "8080"
-	}
-
 	const (
 		readHeaderTimeout = 40 * time.Second
 		readTimeout       = 60 * time.Second
@@ -65,7 +63,7 @@ func (s *Server) Start() error {
 	)
 
 	s.srv = &http.Server{
-		Addr:              ":" + port,
+		Addr:              ":" + strconv.Itoa(s.port),
 		Handler:           s.router,
 		ReadHeaderTimeout: readHeaderTimeout,
 		ReadTimeout:       readTimeout,
@@ -73,7 +71,7 @@ func (s *Server) Start() error {
 		IdleTimeout:       idleTimeout,
 	}
 
-	s.logger.Info("Starting HTTP server", "port", port, "addr", s.srv.Addr)
+	s.logger.Info("Starting HTTP server", "port", s.port, "addr", s.srv.Addr)
 
 	// Start in goroutine but log when actually listening
 	go func() {