feat(discord): self-serve server->org linking with /drb link (#213 item 1)

- c2-core: POST /discord/link-codes (org owner/admin) mints a single-use,
  10-minute code; minting revokes the org's older codes. The bot redeems it
  via POST /discord/guilds/{id}/link/redeem (service key) inside one
  Firestore transaction, so two servers racing for one code cannot both win
  and a server linked to org A refuses org B's code. GET/DELETE
  /discord/linked-guilds lists/unlinks the caller's org's servers; unlink
  stops that server's streams. Rate-limited (mint per user, redeem per
  server and per Discord user); every mint/link/refusal/unlink audited.
- resolve_guild_org: an explicitly unlinked server no longer falls back to
  DISCORD_UNLINKED_GUILD_ORG_ID. Linking away from the fallback org stops
  the streams it had running for that org.
- Discord bot: /drb link code: (Manage Server, enforced at runtime too);
  GatedTree answers every other command in an unlinked server with how to
  link (autocomplete returns nothing). With the default fallback org set,
  nothing is gated — unchanged behaviour.
- Frontend: "Linked Discord servers" card on Settings -> Organization —
  generate code with countdown + copy, polls for the new link, unlink.
- 16 tests: single-use, two-server race, one-server/two-org race, expiry,
  revoked/malformed codes, wrong-org takeover, idempotent relink, rate
  limits, fallback stop, scoped list, unlink without fallback.

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

drb-server-discord-bot/app/commands/drb.py

@@ -0,0 +1,55 @@
+"""/drb — server-level DRB setup. Today: /drb link (server-26#213 item 1)."""
+import discord
+from discord import app_commands
+from discord.ext import commands
+
+from app.internal import guild_gate
+from app.internal.c2_client import c2
+
+
+class DRBCommands(commands.Cog):
+    drb = app_commands.Group(
+        name="drb",
+        description="Set up DRB in this server.",
+        guild_only=True,
+        default_permissions=discord.Permissions(manage_guild=True),
+    )
+
+    def __init__(self, bot: commands.Bot):
+        self.bot = bot
+
+    @drb.command(name="link", description="Link this server to your DRB organization.")
+    @app_commands.describe(code="The code from DRB → Settings → Organization → Discord servers.")
+    async def link(self, interaction: discord.Interaction, code: str):
+        await interaction.response.defer(ephemeral=True)
+        # default_permissions is only the default — a server admin can grant
+        # the command to anyone under Integrations. Linking decides which
+        # organization's radio this whole server gets, so hold the line here.
+        perms = getattr(interaction.user, "guild_permissions", None)
+        if not perms or not perms.manage_guild:
+            await interaction.followup.send("Only members with **Manage Server** can link this server.",
+                                            ephemeral=True)
+            return
+        ok, body = await c2.redeem_guild_link(
+            interaction.guild_id, code, str(interaction.user.id),
+            interaction.guild.name if interaction.guild else None,
+        )
+        guild_gate.forget(interaction.guild_id)
+        radio = self.bot.get_cog("RadioCommands")
+        if radio is not None:
+            radio._overview.pop(interaction.guild_id, None)
+        if not ok:
+            await interaction.followup.send(body.get("detail") or "DRB could not link this server.",
+                                            ephemeral=True)
+            return
+        org = body.get("org_name") or "your DRB organization"
+        if body.get("already_linked"):
+            text = f"This server was already linked to **{org}**. Nothing changed."
+        else:
+            text = (f"This server is now linked to **{org}**.\n"
+                    "`/join`, `/leave` and `/status` now act for that organization only.")
+        await interaction.followup.send(text, ephemeral=True)
+
+
+async def setup(bot: commands.Bot):
+    await bot.add_cog(DRBCommands(bot))