Stop kopia snapshotting comment lines as paths

[mathewcsims]

Last night's backup reported five failing sources and the alert quoted the
SOURCES list's own comment text — so it read as though Super
Productivity's only durable copy had failed to back up.

It hadn't. copyparty/sp-sync and fizzy/storage snapshotted normally in
the same run, seconds apart, with snapshot IDs.

Cause

The loop skipped blank lines but not comments, so kopia was handed
# Super Productivity's synced tasks… as a path:

FAILED: # Super Productivity's synced tasks. This is the ONLY durable copy — the app
  unable to get local filesystem entry: lstat /# Super Productivity's synced...

The SOURCES list is deliberately commented inline — the notes explaining
why those two paths matter sit with the paths — so the first run after
those comments landed failed five times.

Fix

Trim leading whitespace, then skip '' and #*. Covers local-sources.txt
too, which may equally be commented.

Scope — checked, not assumed

• Runs happened every night 5th–9th and all finished cleanly until the
  9th. One night of noise, no missed backups.
• No phantom sources entered the repository — they failed at prepare
  source, so nothing was registered. kopia snapshot list shows 47 sources,
  none beginning /#. The alert's warning that the verifier would flag them
  as stale was moot.
• verify-backups.sh never shared the bug — it derives active sources
  from kopia's own policies, not from this list.

Verification

A real run after the fix: 0 FAILED lines, empty .backup-failures, and
both critical sources snapshotted.

Noted but deliberately left alone: successful runs emit Terminated: 15 to
stderr as each per-source timeout killer is reaped. Pre-existing, harmless,
unrelated.