Discord: check bot permissions before handing a token to a node (server-26#225 S3)

start_stream now asks Discord (cached REST snapshots, S2) whether each bot
it could pick may View, Connect and Speak in the voice channel, before the
lease. Covers all four entry points: bot POST /discord/streams, web picker,
nodes discord_join, and the reconciler's re-place.

- pick_token: denied bots skipped; confirmed-ok sorts before unknown;
  unknown is allowed (fail open); all denied -> missing_permissions naming
  each bot, missing permission, trace reason and fix. The reuse branch
  falls through when its token is denied.
- Bounded: discord_perm_check_concurrency at once, within
  discord_perm_check_budget_seconds; unanswered = unknown.
- Non-voice channel -> 400 bad_channel (not missing_permissions).
- systems/{id}.preferred_token_id is now read (plan finding 1), as a hint.
- Pool bots at discord_pool_bot_guild_cap are never offered as invites;
  pool_full when nothing else is left (owner Q3).
- handle_actual drops cached access on missing_permissions, channel_full,
  not_in_guild, bad_channel.
- New GET /discord/nodes/{node_id}/guilds/{guild_id}/voice-check (org
  member, names only) and GET /discord/guilds/{guild_id}/voice-check
  (service key or admin).
- PUT /tokens/{id}/prefer/{system_id}: org owners for their own systems
  and own BYO bots; shared systems and pool bots stay admin-only (owner Q2).
  Keeps both pointers single-valued; audited.
- Kill switch: DISCORD_PERM_CHECK_ENABLED=false.
- tests/conftest.py: autouse fixture so no test ever reaches discord.com.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

drb-c2-core/app/main.py

@@ -24,6 +24,7 @@ from app.routers import discord_link as discord_link_router
 from app.internal import dynsec
 from app.internal import firestore as fstore
 from app.routers import discord_web
+from app.routers import discord_checks
 
 
 async def _clear_legacy_token_flags():
@@ -164,6 +165,7 @@ app.include_router(discord_web.router)
 # the token-fetch route takes the calling node's own API key.
 app.include_router(discord_router.router)
 app.include_router(discord_link_router.router)  # auth per-endpoint (org owner / bot service key) — server-26#213
+app.include_router(discord_checks.router)  # auth per-endpoint (org member / service key or admin) — server-26#225
 # NOTE: there used to be an app.routers.mqtt_auth router here (an HTTP
 # backend for the mosquitto-go-auth plugin). That plugin's upstream project
 # is archived (no CVE patches) and was rejected for an internet-facing