feat(serenedb): add SereneDB doc-store engine (Go + Python connectors) (#17375)

## What

Adds [**SereneDB**](https://serenedb.com) as a selectable doc-store
engine on **both** RAGFlow paths:
- the **Go** `DocEngine` (`internal/engine/serenedb`), alongside
Elasticsearch and Infinity;
- the **Python** `DocStoreConnection` (`rag/utils/serenedb_conn.py`) +
`DOC_ENGINE=serenedb` registration.

SereneDB is a PostgreSQL-wire engine (DuckDB execution) whose single
inverted index carries **both** a scored text column (`@@`, BM25) and an
IVF vector column (`<#>`, inner product), so hybrid search is one SQL
statement. The Go engine connects with `database/sql` + `lib/pq`
(already a dependency, no new module); the Python connector uses
psycopg2 (already a dependency).

## Storage model

One table per tenant with `kb_id` as a filter column - the
**Elasticsearch / OceanBase** model, not Infinity's per-dataset tables.
This keeps BM25 statistics (IDF, avgdl) computed over the whole tenant
corpus (global IDF). Both connectors use this identical layout, so they
are storage- and retrieval-compatible: `hybrid` proxy routing and
Python↔Go switching are safe. On the Python side the connector is wired
as OceanBase's plain-SQL sibling (chunk_data JSON metadata, inline chunk
vectors, verbatim ES field names); the ES tokenizer path is unchanged.
Metadata stays one table per tenant (`ragflow_doc_meta_<tenant>`).

The query shapes mirror the Python connector, including the five
empirically-found landmines: the scored dictionary needs `frequency +
norm` (else `BM25()` silently returns 0.0), the `@@` query is the
tokenized query, the scored lexical branch matches one column, vectors
use an L2-normalized shadow column with `ip`/`sq8`, and the similarity
threshold goes directly in the ANN scan's `WHERE`. **Minimum engine
version: SereneDB 26.07.4.**

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

docker/docker-compose-base.yml

@@ -100,6 +100,27 @@ services:
       retries: 120
     restart: unless-stopped
 
+  serenedb:
+    profiles:
+      - serenedb
+    image: serenedb/serenedb:26.07.5
+    env_file: .env
+    environment:
+      - POSTGRES_PASSWORD=${SERENEDB_PASSWORD}
+    ports:
+      - ${SERENEDB_PORT}:7890
+    volumes:
+      - serenedb_data:/var/lib/serenedb
+    mem_limit: ${MEM_LIMIT}
+    networks:
+      - ragflow
+    healthcheck:
+      test: ["CMD", "pg_isready", "-h", "127.0.0.1", "-p", "7890", "-U", "postgres"]
+      interval: 10s
+      timeout: 10s
+      retries: 120
+    restart: unless-stopped
+
   oceanbase:
     profiles:
       - oceanbase
@@ -380,6 +401,8 @@ volumes:
     driver: local
   infinity_data:
     driver: local
+  serenedb_data:
+    driver: local
   ob_data:
     driver: local
   seekdb_data: