Backend logging: stderr + configurable rotating file

Add configure_logging() (main.py): always logs to stderr, plus a rotating
file when PIRATS_LOG_FILE is set; level from PIRATS_LOG_LEVEL (default INFO).
Called from both the CLI entrypoint and the app lifespan (idempotent), which
also logs startup/migration/shutdown.

Targeted server-side events for ops/debugging (not every game event): game
created, player joined, player removed (kick/leave), game ended.

NixOS service: new logFile (/var/log/pirats/pirats.log) and logLevel options,
wired to the env vars; LogsDirectory=pirats makes the path writable under the
sandboxed DynamicUser. README documents the env vars/options.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/changelog.js

@@ -6,7 +6,7 @@
 //   - Add a CHANGELOG entry only when a commit changes something players can
 //     see. Skip refactors, tests, and tooling. Keep wording player-facing.
 
-export const VERSION = 8;
+export const VERSION = 9;
 
 // Newest first. Each entry: { version, date: 'YYYY-MM-DD', changes: [string, ...] }.
 export const CHANGELOG = [