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feat(obs): lightweight observability (JSON logs + debug expvar/pprof)

Add dependency-free runtime introspection and contained logging for the
mount daemon (issue #74):

- --log-json global flag: emit structured JSON logs to stderr (systemd/
  journal friendly) instead of the on-disk file.
- mount --debug / --debug-addr: start an opt-in expvar + pprof HTTP
  server on the loopback interface (127.0.0.1:6060 by default) and raise
  the log level to debug.
- New internal/obs package registers live counters (cache, content cache,
  uploads, delta) as lazy expvar.Func and serves /debug/vars + /debug/pprof
  with a bounded ReadHeaderTimeout.
- internal/fs: UploadManager exposes completed/failed/inFlight counters;
  DeltaSync counters moved to atomic.Uint64 with a Counters() accessor so
  the expvar scrape does not race the delta loop; MountConfig gains
  DebugAddr wired in fs.Mount.
- internal/auth/logging.go: rotate the on-disk log by size via lumberjack
  (10MB, at most 5 gzipped backups, 30 days) so it stops growing unbounded
  (user report on 0.1.3). Default level was already Info.
- Docs: MANUAL.md and the EN/ES man pages gain an Observability section.

Verified: make build && make test-unit-short && make lint-all plus the
security lint; all new tests (obs server, rotation config, CLI flags,
upload counters) pass under -race.

Closes #74