Link a Thermograph account to Discord via OAuth2 (#208)

Lets a signed-in user connect their Discord account (OAuth2 identify), storing the
Discord user id that a later feature (DM alerts) will deliver to. Standard
authorization-code flow, all server-side:

- backend/discord_link.py: /discord/link/start redirects to Discord's consent
  screen; /discord/link/callback exchanges the code, reads the Discord user id, and
  stores it; /discord/unlink forgets it. The `state` is signed with the app auth
  secret (stdlib hmac, no new dependency) and carries the Thermograph user id, so a
  callback can't be replayed or bound to another account. Every route requires an
  active session, so linking acts on whoever is actually logged in.
- models.py: User.discord_id (unique, nullable). schemas.py exposes it on UserRead
  so the frontend can show link state.
- app.py: the router under /api/v2/discord.
- account.js: a "Link Discord" / "Unlink Discord" control in the account popover.
- deploy/migrations/001-user-discord-id.sql: the manual column add for the existing
  prod accounts DB. This project has no Alembic — create_all only makes missing
  tables, so an added column needs a hand-applied migration (documented in-file).
- env example: THERMOGRAPH_DISCORD_CLIENT_SECRET + the redirect to register.

Claude-Session: https://claude.ai/code/session_013dRZmX9D3JEntfMKWMTWZ8

deploy/thermograph.env.example

@@ -91,3 +91,8 @@ THERMOGRAPH_BASE_URL=https://thermograph.org
 #THERMOGRAPH_DISCORD_PUBLIC_KEY=
 #THERMOGRAPH_DISCORD_APP_ID=
 #THERMOGRAPH_DISCORD_BOT_TOKEN=
+# Account linking (OAuth2 "identify"): lets a signed-in user connect their Discord
+# account, storing their Discord user id for DM alerts. CLIENT_ID is the same App
+# ID above. The CLIENT_SECRET is from OAuth2 -> Client Secret (a credential). In the
+# portal, add the redirect: https://thermograph.org/api/v2/discord/link/callback
+#THERMOGRAPH_DISCORD_CLIENT_SECRET=