analyze: add --json output

borg analyze currently only produces human-readable text output:

• the set-dedup stats (set / exclusive / rest sizes, see #5741),
• the hot-spot report (directory path -> churn),
• the --by-name repository decomposition table (per-name archives / source / stored / compression, shared, unreferenced).

All other status-type commands (info, repo-info, repo-list, list, diff, prune) already offer --json or --json-lines, so tooling can consume their output without scraping text. analyze is the odd one out, and its output is exactly the kind of data monitoring/agent tooling wants (e.g. a read-only MCP status server, see #9955).

Proposal: add --json to borg analyze, emitting the same numbers the text report is rendered from (raw byte values, not pre-formatted sizes), for both the default mode and --by-name.