docs: update README + api docs + architecture for v2.0.0 feed

README
  - Mention social feed in the one-line description and feature bullets
  - Add relay + feed endpoint tables to the API overview (was
    previously empty on messaging)
  - List media/ package in the repo structure

docs/api/
  - New docs/api/feed.md: full reference for /feed/publish, fetch,
    stats, view, author, timeline, trending, foryou, hashtag; all
    on-chain CREATE_POST / DELETE_POST / FOLLOW / LIKE payloads;
    fee economics; server-side scrubbing contract.
  - docs/api/relay.md rewritten: /relay/broadcast is now the primary
    E2E path with a complete envelope schema; /relay/send kept but
    flagged ⚠ NOT E2E; DELETE /relay/inbox/{id} documented with the
    new Ed25519 signed-auth body.
  - docs/api/README.md index: added feed.md row.

docs/architecture.md
  - L2 Transport layer description updated to include the feed
    mailbox alongside the 1:1 relay mailbox.
  - New "Социальная лента (v2.0.0)" section right after the 1:1
    message flow: ASCII diagram of publish + on-chain commit +
    timeline fetch, economic summary, metadata-scrub summary.

docs/node/README.md
  - Removed stale chan:/chan-member: keys from the BadgerDB schema
    reference; replaced with the v2.0.0 feed keys (post:,
    postbyauthor:, follow:, followin:, like:, likecount:).

docs/update-system.md
  - Example features[] array updated to match the actual node output
    (channels_v1 removed, feed_v2 / media_scrub / relay_broadcast added).

Node feature flags
  - api_well_known_version.go: dropped channels_v1 tag (the
    /api/channels/:id endpoint was removed in the feed refactor);
    added feed_v2, media_scrub, relay_broadcast so clients can
    feature-detect the v2.0.0 surface.
  - Comment example updated channels_v2/v1 → feed_v3/v2.

Client
  - CLIENT_REQUIRED_FEATURES expanded to include the v2.0.0 feature
    flags the client now depends on (feed_v2, media_scrub,
    relay_broadcast); checkNodeVersion() will flag older nodes as
    unsupported and surface an upgrade prompt.

All 7 Go test packages green; tsc --noEmit clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

client-app/lib/api.ts

@@ -480,8 +480,11 @@ export const CLIENT_PROTOCOL_VERSION = 1;
  */
 export const CLIENT_REQUIRED_FEATURES = [
   'chain_id',
+  'feed_v2',           // social feed (v2.0.0) — PostCard, timeline, forYou
   'identity_registry',
+  'media_scrub',       // server-side EXIF strip — we rely on this for privacy
   'onboarding_api',
+  'relay_broadcast',   // /relay/broadcast for E2E envelopes (not /relay/send)
   'relay_mailbox',
   'ws_submit_tx',
 ];