feat: add Qdrant vector database client for JavaScript SDK (#273)

[tarai-dl]

Summary

Implements Qdrant vector database support for the JavaScript SDK, addressing bounty issue #273.

Uses the Qdrant REST API directly as requested (no qdrant npm packages).

Changes

• New module: QdrantClient class in db/src/lib/qdrant-client/QdrantClient.ts
• Exported from: @arakoodev/edgechains.js/db
• Distance metrics: Cosine, Inner Product (Dot), L2 (Euclid) — matching Qdrant's native metrics
• Namespace filtering: Multi-tenant support via Qdrant payload filters
• RRF scoring: Reciprocal Rank Fusion with configurable text/similarity/date weights (matching PostgresClient pattern)
• CRUD operations: Search, create collection, upsert points, delete points, get collection info
• Tests: Comprehensive Jest unit tests with mocks

Usage

import { QdrantClient, QdrantDistanceMetric } from "@arakoodev/edgechains.js/db";

const client = new QdrantClient(
    wordEmbeddings,           // number[][]
    QdrantDistanceMetric.COSINE,
    10,                       // topK
    "my_collection",          // collection name
    "my_namespace",           // namespace filter
    arkRequest,               // RRF weight config
    20,                       // upperLimit
    "localhost",              // Qdrant host
    6333                      // Qdrant port
);

// Search vectors
const results = await client.dbQuery();

// Create collection
await client.createCollection(384); // vector dimension

// Upsert points
await client.upsertPoints([{
    id: "doc1",
    vector: [0.1, 0.2, ...],
    payload: { raw_text: "...", namespace: "my_ns" }
}]);

API Compatibility

The dbQuery() method returns results in the same format as PostgresClient, making it a drop-in replacement:

// Switching from Postgres to Qdrant:
// const client = new PostgresClient(embeddings, metric, topK, probes, table, ns, arkRequest, limit);
const client = new QdrantClient(embeddings, metric, topK, collection, ns, arkRequest, limit, host, port);
const results = await client.dbQuery(); // Same interface

Closes #273

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