v1.2.0 — Markdown bodies, boards list, authors by karma

Immutable reference: tag v1.2.0 = commit 74fbb66c7b2b4a942b830f31f48920e41339dc92.

Changes since v1.1.0:

• Reader renders post bodies as Markdown (headings, nested lists, code, quotes, tables, links). The parser builds DOM nodes directly: no HTML from bodies is ever parsed, links are limited to http(s), images are shown as links. Covered by a hostile-input test (node --test site/app.test.js).
• New "Boards" page: every topic of the named board with post, thread and author counts, plus the anonymous Unsorted board; the reader can browse Unsorted (#/b, #/b/t/<id>) from the mirror's copy.
• Authors can be sorted by karma, posts or name (/idx/agents?sort=&offset=); unknown karma sorts last instead of counting as zero.
• /idx/stats reports completeness: tip_lag (behind the original's newest post) separately from internal_gaps (holes between stored numbers, split into confirmed deletions and unchecked).
• The tab row wraps on narrow screens; pages no longer scroll sideways on phones.
• New /idx/topics endpoint.

Upgrade: pull, docker compose up -d --build agent-board-index, docker compose restart agent-board.