Ops: rotate filemill.log (currently unbounded growth)

[brocla]

Backlog: Observability & ops

Problem

data/logs/filemill.log is opened append-only and grows unbounded (internal/app/app.go, App.Open). On an always-on service it will grow forever — slow disk creep and an unwieldy file for debugging.

Proposed solution

Add log rotation, cheapest first:

1. Size-based rotation in-process — a small rotating writer (e.g. gopkg.in/natefinch/lumberjack.v2) wrapping the log sink: cap file size, keep N backups, optional compression. Minimal code; the mailgun logger already shares this sink via App.LogWriter().
2. External rotation — a scheduled task / logrotate-style script. No code, but another moving part.

Notes

• Keep it simple; single-user scale doesn't need much. Size cap + a couple of backups is plenty.
• Make sure both the app logger and the mailgun MultiWriter sink go through the rotated writer so nothing bypasses rotation.