Add moderation system with deterministic detection and strike ladder

Deterministic-only detection (no LLM): word list with precompiled
leetspeak variation matching, text/image flood, wall of text, repeated
messages (Levenshtein similarity), mention flooding, link rate limiting
for new members, invite flooding, join/leave cycling detection.

Three-strike response ladder: warn + redact → temp mute → permanent ban.
Strikes expire after configurable period. Admin room notifications with
context cards. DMs over public callouts.

Admin commands: !mod warn/mute/unmute/ban/strikes/forgive/history/reload/
status/test. All require ADMIN_USERS membership.

Feature-flagged: disabled by default, set FEATURE_MODERATION=true to
enable. All thresholds configurable via env vars. New member grace
period with stricter thresholds. Word list hot-reload via !mod reload.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/db/db.go

@@ -621,6 +621,31 @@ CREATE TABLE IF NOT EXISTS api_cache (
 	cached_at INTEGER DEFAULT (unixepoch())
 );
 
+-- Moderation: strikes
+CREATE TABLE IF NOT EXISTS mod_strikes (
+	id           INTEGER PRIMARY KEY AUTOINCREMENT,
+	user_id      TEXT NOT NULL,
+	room_id      TEXT NOT NULL,
+	issued_at    DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+	expires_at   DATETIME NOT NULL,
+	reason       TEXT NOT NULL,
+	issued_by    TEXT NOT NULL,
+	active       BOOLEAN NOT NULL DEFAULT TRUE
+);
+CREATE INDEX IF NOT EXISTS idx_mod_strikes_user ON mod_strikes(user_id, issued_at);
+
+-- Moderation: action log
+CREATE TABLE IF NOT EXISTS mod_actions (
+	id           INTEGER PRIMARY KEY AUTOINCREMENT,
+	user_id      TEXT NOT NULL,
+	room_id      TEXT NOT NULL,
+	action       TEXT NOT NULL,
+	reason       TEXT,
+	taken_at     DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+	taken_by     TEXT NOT NULL
+);
+CREATE INDEX IF NOT EXISTS idx_mod_actions_user ON mod_actions(user_id, taken_at);
+
 -- Space groups (rooms with overlapping membership)
 CREATE TABLE IF NOT EXISTS space_groups (
 	room_id    TEXT PRIMARY KEY,