[Feature]: Add SeekDB as an optional derived-index backend

[knqiufan]

Area

architecture methods

Problem or opportunity

EverOS currently uses LanceDB by default and supports Milvus as an optional
derived-index backend. Users who want SeekDB's MySQL-compatible scalar,
full-text, and vector search need a first-class adapter that preserves the
same repository contracts and keeps Markdown as the source of truth.

The adapter should support both an in-process database for local deployments
and a remote seekdb Server or OceanBase endpoint, without adding dependencies
or behavior changes to the default LanceDB installation.

Proposed solution

Add an optional seekdb derived-index backend with:

• configuration and dependency extras for embedded and remote modes;
• implementations of all seven typed derived-index repositories;
• schema creation and strict drift detection for scalar, JSON, full-text, and
  cosine HNSW vector indexes;
• safe SQL rendering, exact datetime conversion, batched writes, native
  pagination, BM25 search, and vector search;
• explicit embedded-directory ownership plus remote connection health checks
  and reconnect-on-disconnect behavior;
• lifecycle and cascade rebuild integration through the existing backend
  ports;
• user-facing configuration and architecture documentation; and
• unit, contract, embedded integration, remote integration, and tiered API
  coverage in CI.

LanceDB remains the default. Switching backends rebuilds the derived data from
Markdown rather than migrating authoritative content.

Alternatives considered

• Keep LanceDB only: this does not serve deployments standardizing on SeekDB
  or OceanBase.
• Use Milvus: it covers remote vector storage but not SeekDB's embedded mode or
  its unified SQL/full-text/vector operational model.
• Add an application-specific storage path outside the index ports: this would
  duplicate routing and leak backend details into memory services.

Additional context

The implementation targets the current pyseekdb 1.4.x API. Embedded support
is available on Linux and macOS; all platforms can use remote mode. The
physical database is treated as rebuildable derived state, consistent with
the existing LanceDB and Milvus backends.