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/providers/providers.go

@@ -14,7 +14,7 @@ import (
 	"time"
 )
 
-const UserAgent = "TopDeck/0.1 (self-hosted favorites list; https://git.iamchrisama.com/cdnutter/topdeck)"
+const UserAgent = "Canon/0.1 (self-hosted favorites list; https://git.iamchrisama.com/cdnutter/canon)"
 
 // Result is one search hit, already shaped like a list item.
 type Result struct {