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/backend/src/shared/config/index.ts

@@ -102,15 +102,6 @@ export const configSchema = z
     AI_LLM_BASE_URL: z.string().url().default("http://127.0.0.1:4014/v1"),
     AI_LLM_MODEL: z.string().default("text"),
     AI_LLM_VISION_MODEL: z.string().optional(),
-    AI_LLM_EMBEDDING_MODEL: z.string().optional(),
-    // Minimum cosine similarity for the public archive semantic search. Lower
-    // = more (noisier) results; raise it to tighten precision. Tuned for a 1B
-    // embedding model — re-tune if the model's dimensionality changes.
-    AI_LLM_EMBEDDING_ARCHIVE_MIN_SIMILARITY: z.coerce
-      .number()
-      .min(0)
-      .max(1)
-      .default(0.6),
     AI_LLM_MAX_CONCURRENT: z.coerce.number().int().positive().default(5),
     AI_LLM_IMAGE_MAX_DIMENSION: z.coerce
       .number()
@@ -179,11 +170,6 @@ export const configSchema = z
       .default("https://api.openai.com/v1"),
     OPENAI_MODERATION_MODEL: z.string().default("omni-moderation-latest"),
 
-    // ── Qdrant (message archive for semantic search) ──────────────────
-    QDRANT_URL: z.string().optional(),
-    QDRANT_API_KEY: z.string().optional(),
-    QDRANT_ARCHIVE_COLLECTION: z.string().default("gmw_message_archive"),
-
     // ── Auto Delete ─────────────────────────────────────────────────────
     AUTO_DELETE_FLAGGED_ENABLED: z
       .string()