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/cacheGuards.test.ts

@@ -1,11 +1,9 @@
 // ═══════════════════════════════════════════════════════════════════════════
-// Semantic two-band acceptance + global exact-cache reuse guard
+// Global exact-cache reuse guard
 // ═══════════════════════════════════════════════════════════════════════════
 // Design (2026-08-24): cache hits may be served MORE aggressively for
 // verdicts that cannot trigger enforcement actions, and NEVER more
-// aggressively for actionable ones. Two layers enforce this:
-//   - isSemanticBandAccepted: similarity thresholds differ by verdict class
-//     (clean band 0.92 default vs strict actionable band 0.97 default).
+// aggressively for actionable ones:
 //   - isGloballyReusableCleanVerdict: context-free (cross-channel) reuse of
 //     the legacy bare key only for clean / flagless / action=none verdicts
 //     with high confidence and bounded age.
@@ -31,64 +29,6 @@ function makeVerdict(
   };
 }
 
-describe("isSemanticBandAccepted", () => {
-  it("accepts a non-actionable clean verdict at the loose clean band", () => {
-    // Default AI_LLM_EMBEDDING_MIN_SIMILARITY_CLEAN = 0.92.
-    expect(isBandAccept(makeVerdict(), 0.93)).toBe(true);
-  });
-
-  it("accepts a clean verdict exactly at the clean band boundary", () => {
-    expect(isBandAccept(makeVerdict({ confidence: 0.99 }), 0.92)).toBe(true);
-  });
-
-  it("rejects a clean verdict below the clean band", () => {
-    expect(isBandAccept(makeVerdict(), 0.91)).toBe(false);
-  });
-
-  it("rejects an actionable flagged verdict between the bands", () => {
-    // 0.93 >= clean band BUT < strict band → must NOT be served.
-    expect(
-      isBandAccept(
-        makeVerdict({ status: "flagged", flags: ["hate_speech"] }),
-        0.93,
-      ),
-    ).toBe(false);
-  });
-
-  it("accepts a flagged verdict at the strict band", () => {
-    expect(
-      isBandAccept(
-        makeVerdict({ status: "flagged", flags: ["hate_speech"] }),
-        0.98,
-      ),
-    ).toBe(true);
-  });
-
-  it("rejects a warn verdict below the strict band", () => {
-    expect(
-      isBandAccept(
-        makeVerdict({ status: "warn", recommendedAction: "warn" }),
-        0.96,
-      ),
-    ).toBe(false);
-  });
-
-  it("treats a clean verdict WITH flags as actionable (strict band)", () => {
-    expect(isBandAccept(makeVerdict({ flags: ["borderline"] }), 0.93)).toBe(
-      false,
-    );
-  });
-
-  it("treats a clean verdict with a non-none action as actionable", () => {
-    expect(
-      isBandAccept(makeVerdict({ recommendedAction: "review" }), 0.93),
-    ).toBe(false);
-  });
-});
-
-// Import indirection so the describe block reads cleanly.
-import { isSemanticBandAccepted as isBandAccept } from "../src/modules/ai-moderation/textCacheStore.js";
-
 describe("isGloballyReusableCleanVerdict", () => {
   it("accepts a fresh, confident, flagless clean verdict", () => {
     const v = makeVerdict({ confidence: 0.9 });