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.

docs/systems/database.md

@@ -105,6 +105,7 @@ PK: (spaceId, userId)
 | width | integer | | Image/video pixel width |
 | height | integer | | Image/video pixel height |
 | duration | real | | Audio/video seconds |
+| playable | integer (boolean) | NULL | Video web-playability tri-state: 1 = browser-decodable, 0 = known-undecodable (e.g. HEVC .mov), NULL = unknown/non-video. See uploads.md §3. |
 | sourceUrl | text | | Remote URL (federation) |
 | federationStatus | text | | local/remote/remote_partial |
 | federationMeta | text | | JSON rejection info |