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/routes/federation.ts

@@ -2911,6 +2911,7 @@ export async function federationRoutes(app: FastifyInstance): Promise<void> {
           width: a.width ?? undefined,
           height: a.height ?? undefined,
           duration: a.duration ?? undefined,
+          playable: a.playable ?? null,
           thumbnailFilename: a.thumbnailFilename ?? undefined,
           sourceUrl: `${localOrigin}/api/uploads/${a.filename}`,
         }));
@@ -3656,6 +3657,7 @@ async function processCreateEvent(
           width: attachment.width ?? null,
           height: attachment.height ?? null,
           duration: attachment.duration ?? null,
+          playable: attachment.playable ?? null,
           thumbnailFilename: null,  // Don't copy source thumbnail — it doesn't exist locally
           sourceUrl: attachment.sourceUrl,
           createdAt: now,
@@ -3830,6 +3832,7 @@ function processUpdateEvent(
       width: a.width,
       height: a.height,
       duration: a.duration,
+      playable: a.playable ?? null,
       createdAt: a.createdAt,
     }));