refactor(ai): remove semantic embedding cache + Qdrant vector store

Hapus seluruh fitur embedding/Qdrant (tidak dipakai lagi):

- gateway: drop embeddingClient.ts, qdrantClient.ts, archiveEmbedder.ts
  dan tes qdrantEnsure.test.ts; moderationOrchestrator kembali ke
  exact-hash cache -> LLM (tanpa phase-2 semantic lookup); textCacheStore
  kehilangan findSimilarTextModeration / parseQdrantVerdict /
  isSemanticBandAccepted / upsertBareKeyToQdrant; cache-prune hanya
  menyapu Postgres.
- backend: drop embed.ts + qdrant.ts, endpoint messages.semanticSearch
  dan schema/type terkait; kolom embedding dilepas dari schema
  text_analysis_cache.
- frontend: hapus toggle EXACT/SEMANTIC, hook useSemanticSearch,
  API client + tipe SemanticSearchResult.
- config: buang AI_LLM_EMBEDDING_* dan QDRANT_* (env + .env.example).
- docs: ARCHITECTURE.md / AGENTS.md / README.md / diagram arsitektur
  disesuaikan (LLM caller - vision, cache = exact-hash saja).

Verifikasi: tsc 0 (backend, gateway, frontend); bun test 135 pass +
37 pass, 0 fail; biome 0 error.

services/discord-gateway/tests/storedStatusNormalization.test.ts

@@ -6,10 +6,10 @@
 // ["conflict_instigation"]) fell into the legacy `flags.length === 0 ?
 // clean : flagged` branch and was read back as FLAGGED. Downstream this
 // broke auto-delete eligibility gating and mislabelled warnings on the
-// dashboard. parseQdrantVerdict had the same narrowing (warn → clean).
+// dashboard.
 //
-// Fix: normalizeStoredStatus() accepts the full clean/warn/flagged union in
-// BOTH readers; unknown/legacy values still derive from flags.
+// Fix: normalizeStoredStatus() accepts the full clean/warn/flagged union;
+// unknown/legacy values still derive from flags.
 import { describe, expect, it } from "vitest";
 import { normalizeStoredStatus } from "../src/modules/ai-moderation/textCacheStore.js";