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>

docs/node/README.md

@@ -147,8 +147,12 @@ txchron:<block20d>:<seq04d>   → tx_id (recent-tx index)
 balance:<pubkey>              → uint64 (µT)
 stake:<pubkey>                → uint64 (µT)
 id:<pubkey>                   → JSON RegisterKeyPayload
-chan:<channelID>              → JSON CreateChannelPayload
-chan-member:<ch>:<pub>        → ""
+post:<postID>                 → JSON PostRecord (v2.0.0 social feed)
+postbyauthor:<pub>:<ts>:<id>  → "" (chrono index, newest-first scan)
+follow:<A>:<B>                → "" (A follows B)
+followin:<B>:<A>              → "" (reverse index for Followers())
+like:<postID>:<liker>         → "" (presence)
+likecount:<postID>            → uint64 (cached counter, O(1) reads)
 contract:<contractID>         → JSON ContractRecord
 cstate:<contractID>:<key>     → bytes
 clog:<ct>:<block>:<seq>       → JSON ContractLogEntry