Rename project from TopDeck to Canon

- Module github.com/cdnutter/canon, binary/command cmd/canon, Docker
  image and compose project "canon"
- Environment variables now use the CANON_ prefix
- Default site title is "<Your name>'s Canon" (or "Canon" with no name);
  a Site title set in Settings still overrides it
- Existing topdeck.db (and WAL files) is renamed to canon.db on start
- README: upgrade notes, including moving the old compose volume

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/server/server.go

@@ -18,16 +18,16 @@ import (
 	"strings"
 	"time"
 
-	"github.com/cdnutter/topdeck/internal/auth"
-	"github.com/cdnutter/topdeck/internal/config"
-	"github.com/cdnutter/topdeck/internal/imgcache"
-	"github.com/cdnutter/topdeck/internal/providers"
-	"github.com/cdnutter/topdeck/internal/store"
-	"github.com/cdnutter/topdeck/web"
+	"github.com/cdnutter/canon/internal/auth"
+	"github.com/cdnutter/canon/internal/config"
+	"github.com/cdnutter/canon/internal/imgcache"
+	"github.com/cdnutter/canon/internal/providers"
+	"github.com/cdnutter/canon/internal/store"
+	"github.com/cdnutter/canon/web"
 )
 
 const (
-	sessionCookie = "topdeck_session"
+	sessionCookie = "canon_session"
 	sessionTTL    = 30 * 24 * time.Hour
 )
 
@@ -240,7 +240,7 @@ func (s *Server) requireUser(h http.HandlerFunc) http.HandlerFunc {
 // preflight, which blocks CSRF.
 func (s *Server) requireAPI(h http.HandlerFunc) http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
-		if r.Header.Get("X-TopDeck") != "1" || !s.sameOrigin(r) {
+		if r.Header.Get("X-Canon") != "1" || !s.sameOrigin(r) {
 			writeError(w, http.StatusForbidden, "Missing request header")
 			return
 		}