feat(backend): Discord OAuth login with DB-backed sessions (#23) (#31)

Implements #23 per ADR-0002.

- Discord authorization code grant (identify + guilds.members.read), form-encoded token exchange
- Guild membership gate via the single-guild endpoint; optional DISCORD_REQUIRED_ROLE (empty default)
- Owner Discord ID is the only identity allowed to sign in
- Sessions are DB rows with opaque random ids; cookie carries only the id; expiry enforced; delete = revoke
- HMAC session signing, derived key, and WEB_PASSWORD removed; no replacement signing secret
- Login rate limiting preserved on the callback
- Full flow tested through the real router against a local Discord stub (DISCORD_API_BASE)
- Env: DISCORD_CLIENT_ID/_CLIENT_SECRET/_GUILD_ID/_REQUIRED_ROLE/_API_BASE/_REDIRECT_URI; docs updated

go test ./... passes.

Reviewed-on: #31
Co-authored-by: Sulthan Zaki <sultankiki05@gmail.com>
Co-committed-by: Sulthan Zaki <sultankiki05@gmail.com>

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;
@@ -810,7 +790,13 @@ button { cursor: pointer; }
 .login-card button:hover {
   background: var(--ember);
   border-color: var(--ember);
-  color: #fff;
+  color: var(--ember-ink);
+}
+.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