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>

internal/engine/serenedb/sql.go

@@ -0,0 +1,58 @@
+//
+//  Copyright 2026 The InfiniFlow Authors. All Rights Reserved.
+//
+//  Licensed under the Apache License, Version 2.0 (the "License");
+//  you may not use this file except in compliance with the License.
+//  You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+//  Unless required by applicable law or agreed to in writing, software
+//  distributed under the License is distributed on an "AS IS" BASIS,
+//  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+//  See the License for the specific language governing permissions and
+//  limitations under the License.
+//
+
+package serenedb
+
+import (
+	"context"
+	"fmt"
+	"regexp"
+	"strings"
+)
+
+const runSQLDefaultLimit = 1024
+
+var (
+	selectPrefixRe = regexp.MustCompile(`(?i)^(select|with)\b`)
+	hasLimitRe     = regexp.MustCompile(`(?i)\blimit\b`)
+)
+
+// boundRunSQL trims a statement and appends a default LIMIT to unbounded
+// SELECT/WITH queries so the text-to-SQL path cannot stream an entire table.
+// Pure for tests.
+func boundRunSQL(sqlText string, limit int) string {
+	txt := strings.TrimSpace(sqlText)
+	txt = strings.TrimRight(txt, ";")
+	if limit > 0 && selectPrefixRe.MatchString(txt) && !hasLimitRe.MatchString(txt) {
+		txt = fmt.Sprintf("%s LIMIT %d", txt, limit)
+	}
+	return txt
+}
+
+// RunSQL executes a text-to-SQL query directly against SereneDB and returns the
+// rows as maps. SereneDB speaks SQL natively, so unlike the Infinity engine no
+// psql subprocess or field-alias rewrite is needed.
+func (e *serenedbEngine) RunSQL(ctx context.Context, tableName string, sqlText string, kbIDs []string, format string) ([]map[string]interface{}, error) {
+	query := boundRunSQL(sqlText, runSQLDefaultLimit)
+	rows, err := e.queryMaps(ctx, query)
+	if err != nil {
+		return nil, fmt.Errorf("serenedb: run sql: %w", err)
+	}
+	if rows == nil {
+		rows = []map[string]interface{}{}
+	}
+	return rows, nil
+}