Capture member/filter/spam events for the dashboard (Phase 6b)

Light up the moderation dashboard's previously-empty widgets by persisting the
event streams the bot only reacted to in-memory before.

Schema (bot-owned)
- member_events: join/leave, with invite_code/inviter_* for best-effort invite
  attribution on joins
- filter_hits: word / foreign-invite filter deletions (matched + action_taken)
- spam_hits: rate_limit / mass_mention / mass_emoji detections

Bot
- new models memberEvents/filterHits/spamHits
- guildMemberAdd records the join with invite attribution; new inviteTracker.js
  keeps an invite-use cache (GuildInvites intent + inviteCreate/inviteDelete) and
  diffs it on join to find which invite was used — best-effort, never blocks
  auto-role
- new guildMemberRemove records leaves
- messageFilter records filter/spam hits alongside the existing warn/mute;
  inviteFilter now returns the offending code; detectSpam identifies which spam
  rule tripped (preserving the rate-limit-first side-effect order)
- mod_actions still logs the resulting warn/mute — the new tables are additive

Server
- summary extended with joins/leaves/invite_joins/filter_hits/spam_hits per window
- new feeds: /api/v1/admin/moderation/{members,filter-hits,spam-hits}

Client
- overview now shows 8 tiles (mod actions + joins/leaves/filter/spam, joins tile
  notes "N via invite") plus an Events panel with Members/Filter/Spam tabs;
  removed the coming-soon note

Verified: 119 server unit tests, client build, 14-check DB-backed smoke, and a
browser click-through of every tile and events tab (incl. invite attribution).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019rao86n5cXpwAyjdBFEshV

bot/src/model/memberEvents.js

@@ -0,0 +1,14 @@
+// Guild member join/leave events (Phase 6b). Bot-owned; the site reads these for
+// the moderation dashboard's members feed + invite-usage view. Schema in
+// server/db/schema.sql (member_events).
+const db = require('../db')
+
+async function record({ guildId, eventType, discordUserId, username, inviteCode, inviterId, inviterTag }) {
+  await db.query(
+    `INSERT INTO member_events (guild_id, event_type, discord_user_id, username, invite_code, inviter_id, inviter_tag)
+     VALUES (?, ?, ?, ?, ?, ?, ?)`,
+    [guildId, eventType, discordUserId, username || null, inviteCode || null, inviterId || null, inviterTag || null],
+  )
+}
+
+module.exports = { record }