fix(uploads): graceful fallback for browser-unplayable video (HEVC .mov)

macOS screen recordings are HEVC inside a .mov container, which Chromium,
Firefox and stock Electron can't decode. The file uploaded fine and a
server-side ffmpeg poster was generated, but inline <video> playback failed
silently — stuck at 0:00 with no error, since AttachmentRenderer had no error
handling. Root cause: the system had no concept of web-playability.

Server detects, client degrades:
- mediaPlayable.ts: classifyVideoPlayable(mimetype, codec) — tri-state
  (false = known-undecodable e.g. HEVC/ProRes, true = web codec in web
  container, null = unknown/optimistic). Never widens `false` beyond codecs
  that fail everywhere, so ffmpeg-less instances keep prior behaviour.
- probeMediaMeta now captures the video codec_name; the upload finish hook
  stores the verdict in the new attachments.playable column (migration 0007).
- Flag propagated through every serializer: space messages, DMs, WS, and
  federation relay (outbound + inbound) — federation-compatible.
- VideoAttachment component: playable===false renders a download card (poster
  + "Can't play here — download" + name/duration/size) with no dead-player
  flash; otherwise plays inline with an onError fallback to the same card.

Specs updated: uploads.md, database.md, federation.md.

packages/server/src/db/schema.ts

@@ -102,6 +102,10 @@ export const attachments = sqliteTable('attachments', {
   width: integer('width'),
   height: integer('height'),
   duration: real('duration'),
+  // Tri-state web-playability for video attachments: 1 = decodable in a
+  // browser <video>, 0 = known-undecodable (e.g. HEVC .mov), NULL = unknown
+  // (codec unprobed / non-video). Drives the client's download fallback.
+  playable: integer('playable', { mode: 'boolean' }),
   sourceUrl: text('source_url'),
   federationStatus: text('federation_status'),
   federationMeta: text('federation_meta'),