feat(web/beads): filters for the parade board

Add a filter bar to the beads board: text search (id/title), issue type,
priority, assignee, and label. Filters are query-param based (GET, sticky
across submits) and applied server-side before lane bucketing, so the marquee
counts reflect the filtered set. Dropdown options are collected from the full
issue set (they don't shrink as filters narrow the board); a Clear link shows
when any filter is active. Verified on the 143-issue tarantool-etcd board
(type=bug → 30).

feat(web/beads): epic view with subtask rollup, tabbed Comments/History

Three additions to the beads issue detail:

- Epic mode: when the viewed issue is issue_type=epic, render a Subtasks panel
  — its parent-child children (deps pointing at the epic) with a done/total
  progress meter, sorted open-work-first with closed sinking to the bottom.

- Activity is now a two-tab strip (pure-CSS, no JS): Comments keeps the plain
  thread; History merges comments and the audit-log events table into one
  time-sorted timeline with humanized change lines ("changed status to
  in_progress", "updated priority to 0", "closed the issue" + reason).

- Close reason moved from the top metadata table to its own block at the very
  bottom, after the activity — it reads as a closing note, not a header field.

Verified end-to-end against a real 143-issue beads store (tarantool-etcd) via
the env-guarded TestRealBeadsStore smoke test; unit-covered with a dedicated
epic/history fixture.