feat(glossary): persist resolved definitions in Postgres + harden live SearXNG lookups

- Add term_glossary_cache table + migration 0014: resolved definitions are
  stored permanently (definitions rarely change); misses stay ephemeral in
  Redis/LRU with 1h TTL so transient failures get retried
- Lookup flow: LRU -> Redis -> Postgres (permanent) -> live SearXNG; DB hits
  re-warm the fast caches; stale Redis miss sentinels no longer shadow DB
- Rate-limit-aware live lookups: concurrency 2 + stagger, retry once on empty
  results, strict definition filter (Wikipedia preferred, rejects
  disambiguation/ads/translate-homepages)
- Make SEARXNG_BASE_URL configurable via env (default unchanged)

services/discord-gateway/drizzle/migrations/0014_add_term_glossary_cache.sql

@@ -0,0 +1,9 @@
+CREATE TABLE IF NOT EXISTS "term_glossary_cache" (
+	"term" text PRIMARY KEY NOT NULL,
+	"definition" text NOT NULL,
+	"source_url" text DEFAULT '' NOT NULL,
+	"resolved_at" bigint NOT NULL,
+	"hit_count" integer DEFAULT 0 NOT NULL
+);
+--> statement-breakpoint
+CREATE INDEX IF NOT EXISTS "idx_term_glossary_cache_resolved_at" ON "term_glossary_cache" USING btree ("resolved_at");

services/discord-gateway/drizzle/migrations/meta/_journal.json

@@ -99,6 +99,13 @@
       "when": 1785551832190,
       "tag": "0013_rename_mascot_chat_to_chatbot",
       "breakpoints": true
+    },
+    {
+      "idx": 14,
+      "version": "7",
+      "when": 1785621600000,
+      "tag": "0014_add_term_glossary_cache",
+      "breakpoints": true
     }
   ]
 }