feat(backend): Discord OAuth login with DB-backed sessions (#23)

The browser UI signs in with a Discord authorization code grant
(identify + guilds.members.read) instead of a shared password. Guild
membership is the gate; the owner's Discord ID is the only identity
allowed in while registration is closed. Sessions become rows in a
sessions table with opaque random ids — the cookie carries only the id,
looked up and expiry-checked per request — so deleting a row revokes a
session. HMAC cookie signing, its derived key, and WEB_PASSWORD are
gone, and no replacement signing secret is introduced (ADR-0002).

Discord's API base is configurable (DISCORD_API_BASE); the full flow is
tested through the real router against a local stub, including the
form-encoded token exchange Discord rejects if sent as JSON.

backend/internal/web/static/style.css

@@ -774,26 +774,6 @@ button { cursor: pointer; }
   filter: drop-shadow(0 0 34px var(--ember-wash)) drop-shadow(0 18px 24px rgba(0,0,0,.5));
 }
 .login-card form { display: flex; flex-direction: column; gap: 18px; }
-.login-card label {
-  font: 500 10px/1 var(--font-mono);
-  letter-spacing: .16em;
-  text-transform: uppercase;
-  color: var(--mute);
-}
-.login-card input {
-  width: 100%;
-  height: 54px;
-  margin-top: 9px;
-  padding: 0 2px;
-  border: none;
-  border-bottom: 1px solid var(--field-line);
-  background: transparent;
-  color: var(--paper);
-  font: 500 20px var(--font-mono);
-  letter-spacing: .16em;
-  outline: none;
-}
-.login-card input:focus { border-bottom-color: var(--paper); }
 .login-card .error {
   margin: 0;
   min-height: 20px;
@@ -812,6 +792,12 @@ button { cursor: pointer; }
   border-color: var(--ember);
   color: #fff;
 }
+.login-card .login-note {
+  margin: 14px 0 0;
+  text-align: center;
+  font: 400 12px/1.4 var(--font-body);
+  color: var(--mute);
+}
 
 /* ---- laptop and up: the whole sheet is drawn 20% larger, which is what
    reading it at 120% zoom on a 1920-wide screen was doing by hand. Everything