fix: change appname to neoirc and default DB to /var/lib/neoirc/state.db

- Change Appname from "chat" to "neoirc" in cmd/chatd/main.go
- Change default DBURL from file:./data.db to file:///var/lib/neoirc/state.db
  in both internal/config/config.go and internal/db/db.go fallback
- Create /var/lib/neoirc/ directory in Dockerfile with proper ownership
- Update README.md to reflect new defaults (DBURL, docker run example,
  SQLite backup/location docs)
- Remove stale data.db reference from Makefile clean target

The DB path remains configurable via DBURL environment variable.

Closes #44

cmd/chatd/main.go

@@ -15,7 +15,7 @@ import (
 
 var (
 	// Appname is the application name, set at build time.
-	Appname = "chat" //nolint:gochecknoglobals
+	Appname = "neoirc" //nolint:gochecknoglobals
 
 	// Version is the application version, set at build time.
 	Version string //nolint:gochecknoglobals