feat: filter feed directory by topic/language and export OPML

[gildesmarais]

What changed

• Filter the feed directory UI by catalog directory.topics and channel.language (src/components/FeedDirectory.astro, src/components/feed-directory.js).
• Export the visible filtered set as OPML.
• Refresh src/data/configs.json / src/data/loadConfigs.ts and bin/data-update for the new metadata.
• Point Gemfile at html2rss feat/directory-topics and html2rss-configs feat/directory-topics so CI resolves unreleased catalog fields.

Why

Ship the user-facing half of feed-directory topic/language filtering requested in #135. Data and schema come from the core + configs PRs.

Risk

• Blocked on feat: require directory.topics on feed configs html2rss#458 and feat: add directory topics to curated configs html2rss-configs#310 — merge those first (core → configs → docs).
• Gemfile pins track feature branches until releases exist; revert to default-branch/git tip (or released gems) after merge/release.
• Front-end filter logic: AND semantics across selected topics/languages — wrong filters are UX-only, not build-breaking once data is present.

Review map

1. src/components/feed-directory.js — filter + OPML export behavior.
2. src/components/FeedDirectory.astro — UI wiring.
3. Gemfile / Gemfile.lock — temporary branch pins.
4. src/data/configs.json — generated metadata payload (skim).

Validation

• not run in this open step beyond bundle lock after the Gemfile pin change

Ticket

Fixes #135

Depends on

• feat: require directory.topics on feed configs html2rss#458
• feat: add directory topics to curated configs html2rss-configs#310

Merge order

core → configs → docs. Merge this last.