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

client/src/api/client.js

@@ -131,6 +131,28 @@ export const api = {
       return req(`/admin/moderation/recent${s ? `?${s}` : ''}`)
     },
     modSearch: (q) => req(`/admin/moderation/search?q=${encodeURIComponent(q)}`),
+    modMembers: (params = {}) => {
+      const qs = new URLSearchParams()
+      if (params.type) qs.set('type', params.type)
+      if (params.limit) qs.set('limit', params.limit)
+      if (params.offset) qs.set('offset', params.offset)
+      const s = qs.toString()
+      return req(`/admin/moderation/members${s ? `?${s}` : ''}`)
+    },
+    modFilterHits: (params = {}) => {
+      const qs = new URLSearchParams()
+      if (params.limit) qs.set('limit', params.limit)
+      if (params.offset) qs.set('offset', params.offset)
+      const s = qs.toString()
+      return req(`/admin/moderation/filter-hits${s ? `?${s}` : ''}`)
+    },
+    modSpamHits: (params = {}) => {
+      const qs = new URLSearchParams()
+      if (params.limit) qs.set('limit', params.limit)
+      if (params.offset) qs.set('offset', params.offset)
+      const s = qs.toString()
+      return req(`/admin/moderation/spam-hits${s ? `?${s}` : ''}`)
+    },
     modUser: (discordId) => req(`/admin/moderation/user/${discordId}`),
     modUserActions: (discordId, params = {}) => {
       const qs = new URLSearchParams()