Add `stats --json` for scripting

[ssrajadh]

Problem

sentrysearch/cli.py:1224-1249 only emits human-formatted text. Users wiring SentrySearch into shell scripts or other tools have no machine-readable output to parse, and the [missing] marker is mixed into the file string rather than being a separate field.

Suggested fix

Add --json to the stats command. When set, print a single JSON object:

{
  "backend": "gemini",
  "model": null,
  "total_chunks": 1234,
  "unique_source_files": 42,
  "source_files": ["/abs/path/a.mp4", "/abs/path/b.mp4"],
  "missing_files": ["/abs/path/gone.mp4"]
}

Keep missing_files as its own key (rather than " [missing]" suffixes on source_files) so downstream tools can act on it cleanly. The human path stays exactly as it is today.

Acceptance

• sentrysearch stats --json | jq .total_chunks returns the chunk count.
• sentrysearch stats (no flag) output is unchanged.
• Empty-index case still works (--json returns the same schema with total_chunks: 0).

Scope

Touches only stats in sentrysearch/cli.py plus a test in tests/test_cli.py.