feat: add in-memory vector search plugin with multilingual support

vector-search-memory/README.md

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+# Apache Answer Plugin: In-Memory Vector Search
+
+A lightweight, pure-Go, in-memory vector search plugin for [Apache Answer](https://github.com/apache/answer). Designed for **development and testing** -- no external services, no Docker, no CGo required.
+
+## When to Use This Plugin
+
+- **Development** -- quickly test semantic search without setting up a database
+- **Testing** -- verify vector search integration in CI/CD pipelines
+- **Small demos** -- run Answer with vector search on a single machine with zero setup
+
+**Not recommended for production** -- all data is stored in memory and lost on restart.
+
+## Installation
+
+Build Apache Answer with this plugin:
+
+```bash
+./answer build --with github.com/apache/answer-plugins/vector-search-memory
+```
+
+No special environment variables needed. Works with `CGO_ENABLED=0`.
+
+## Configuration
+
+After enabling the plugin in the Admin UI (**Admin > Plugins > Vector Search**), configure the following fields:
+
+| Field | Description | Example |
+|---|---|---|
+| **Embedding API Host** | OpenAI-compatible API base URL | `https://api.openai.com` |
+| **Embedding API Key** | API key for the embedding service | `sk-...` |
+| **Embedding Model** | Model name for generating embeddings | `text-embedding-3-small` |
+| **Embedding Level** | `question` embeds question + all answers + comments together; `answer` embeds each answer separately | `question` |
+| **Similarity Threshold** | Minimum cosine similarity score (0-1). Default `0` means no filtering | `0.5` |
+
+No connection endpoint or database path is needed -- everything runs in-process.
+
+## How It Works
+
+- Stores all document embeddings in a Go `map[string]*document` guarded by `sync.RWMutex`
+- Search performs brute-force cosine similarity over all stored vectors
+- Embedding dimensions are auto-detected from the configured model
+- Changing the embedding model clears all stored documents (since dimensions may differ)
+- A full sync of all questions/answers is triggered when the plugin starts
+
+## Limitations
+
+- **No persistence** -- data is lost when Answer restarts
+- **No scalability** -- brute-force search is O(n) per query; suitable for thousands of documents, not millions
+- **Memory usage** -- each document stores a full embedding vector in RAM
+
+For production use, consider [pgvector](../vector-search-pgvector/), [Elasticsearch](../vector-search-elasticsearch/), [Weaviate](../vector-search-weaviate/), [Milvus](../vector-search-milvus/), [Qdrant](../vector-search-qdrant/), or [ChromaDB](../vector-search-chromadb/).
+
+## License
+
+[Apache License 2.0](https://www.apache.org/licenses/LICENSE-2.0)

vector-search-memory/go.mod

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+module github.com/apache/answer-plugins/vector-search-memory
+
+go 1.23.0
+
+require (
+	github.com/apache/answer v1.7.0
+	github.com/apache/answer-plugins/util v1.0.3-0.20250107030257-cf94ebc70954
+	github.com/segmentfault/pacman v1.0.5-0.20230822083413-c0075a2d401f
+)
+
+require (
+	github.com/LinkinStars/go-i18n/v2 v2.2.2 // indirect
+	github.com/asaskevich/govalidator v0.0.0-20230301143203-a9d515a09cc2 // indirect
+	github.com/aymerick/douceur v0.2.0 // indirect
+	github.com/bytedance/sonic v1.12.2 // indirect
+	github.com/bytedance/sonic/loader v0.2.0 // indirect
+	github.com/cloudwego/base64x v0.1.4 // indirect
+	github.com/cloudwego/iasm v0.2.0 // indirect
+	github.com/gabriel-vasile/mimetype v1.4.5 // indirect
+	github.com/gin-contrib/sse v0.1.0 // indirect
+	github.com/gin-gonic/gin v1.10.0 // indirect
+	github.com/go-playground/locales v0.14.1 // indirect
+	github.com/go-playground/universal-translator v0.18.1 // indirect
+	github.com/go-playground/validator/v10 v10.22.1 // indirect
+	github.com/goccy/go-json v0.10.3 // indirect
+	github.com/golang/snappy v0.0.4 // indirect
+	github.com/google/wire v0.5.0 // indirect
+	github.com/gorilla/css v1.0.1 // indirect
+	github.com/json-iterator/go v1.1.12 // indirect
+	github.com/klauspost/cpuid/v2 v2.2.8 // indirect
+	github.com/leodido/go-urn v1.4.0 // indirect
+	github.com/mattn/go-isatty v0.0.20 // indirect
+	github.com/microcosm-cc/bluemonday v1.0.27 // indirect
+	github.com/modern-go/concurrent v0.0.0-20180306012644-bacd9c7ef1dd // indirect
+	github.com/modern-go/reflect2 v1.0.2 // indirect
+	github.com/pelletier/go-toml/v2 v2.2.3 // indirect
+	github.com/segmentfault/pacman/contrib/i18n v0.0.0-20230822083413-c0075a2d401f // indirect
+	github.com/syndtr/goleveldb v1.0.0 // indirect
+	github.com/tidwall/gjson v1.17.3 // indirect
+	github.com/tidwall/match v1.1.1 // indirect
+	github.com/tidwall/pretty v1.2.1 // indirect
+	github.com/twitchyliquid64/golang-asm v0.15.1 // indirect
+	github.com/ugorji/go/codec v1.2.12 // indirect
+	github.com/yuin/goldmark v1.7.4 // indirect
+	golang.org/x/arch v0.10.0 // indirect
+	golang.org/x/crypto v0.36.0 // indirect
+	golang.org/x/net v0.38.0 // indirect
+	golang.org/x/sys v0.31.0 // indirect
+	golang.org/x/text v0.23.0 // indirect
+	google.golang.org/protobuf v1.34.2 // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
+	sigs.k8s.io/yaml v1.4.0 // indirect
+	xorm.io/builder v0.3.13 // indirect
+	xorm.io/xorm v1.3.2 // indirect
+)