feat: add 6 vector-search backend plugins

vector-search-chromadb/README.md

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+# Apache Answer Plugin: ChromaDB Vector Search
+
+This plugin enables semantic/vector search in [Apache Answer](https://github.com/apache/answer) using [ChromaDB](https://www.trychroma.com/).
+
+## Prerequisites
+
+- ChromaDB instance (self-hosted)
+- An OpenAI-compatible embedding API (e.g., OpenAI, Azure OpenAI, or any compatible provider)
+
+## Installation
+
+Build Apache Answer with this plugin:
+
+```bash
+./answer build --with github.com/apache/answer-plugins/vector-search-chromadb
+```
+
+## Configuration
+
+After enabling the plugin in the Admin UI (**Admin > Plugins > Vector Search**), configure the following fields:
+
+| Field | Description | Example |
+|---|---|---|
+| **ChromaDB Endpoint** | ChromaDB HTTP API URL | `http://localhost:8000` |
+| **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` |
+
+## How It Works
+
+- **Embedding dimensions** are auto-detected from the configured model. No manual dimension configuration is needed.
+- On first configuration, the plugin creates a collection `answer_vector_embeddings` with `hnsw:space` set to cosine.
+- Uses the ChromaDB REST API directly (no external Go SDK required).
+- ChromaDB cosine distance (0 = identical, 2 = opposite) is converted to a similarity score (1.0 = identical, 0.0 = opposite).
+- A full sync of all questions/answers is triggered when the plugin starts.
+
+## Running ChromaDB Locally
+
+Using Docker:
+
+```bash
+docker run -p 8000:8000 chromadb/chroma
+```
+
+The REST API will be available at `http://localhost:8000`.
+
+## License
+
+[Apache License 2.0](https://www.apache.org/licenses/LICENSE-2.0)