docs: add German (DE) documentation + update all docs for Sessions 10-12

New German documentation (docs/de/):
- index.md, quickstart.md, installation.md
- baraql.md, graph.md, vector.md, mcp.md

Updated English documentation:
- changelog.md: all Sessions 10-12 features
- graph.md: SQL GRAPH_TABLE, CREATE GRAPH, all 8 algorithms, Cypher, similarity_nodes, node2vec
- vector.md: hybrid RAG, chunk(), embed_text(), auto-embed, nl_to_sql(), schema_prompt()
- baraql.md: new AI & Cross-Modal Functions section, updated keyword tables
- mcp.md: MCP Server documentation (new file)
- index.md: added German (DE) language link

docs/en/mcp.md

@@ -0,0 +1,110 @@
+# MCP Server (Model Context Protocol)
+
+BaraDB includes a built-in MCP server that enables AI agents (Claude, Cursor, etc.)
+to interact with the database directly.
+
+## Quick Start
+
+```bash
+./build/baramcp --data-dir ./data
+```
+
+Starts in STDIO mode, accepting JSON-RPC 2.0 messages on stdin.
+
+## Available Tools
+
+### 1. `query` — SQL Execution
+
+```json
+{
+  "name": "query",
+  "arguments": {
+    "sql": "SELECT * FROM users WHERE age > ?",
+    "params": [25],
+    "tenant_id": "company-a",
+    "user_id": "alice"
+  }
+}
+```
+
+Parameterized queries using `?` placeholders. Multi-tenant via `tenant_id` and `user_id`.
+
+### 2. `vector_search` — Semantic Search
+
+```json
+{
+  "name": "vector_search",
+  "arguments": {
+    "table": "docs",
+    "column": "embedding",
+    "query_vector": [0.1, 0.2, 0.3],
+    "k": 5,
+    "metric": "cosine",
+    "filter_column": "category",
+    "filter_value": "news",
+    "tenant_id": "company-a"
+  }
+}
+```
+
+Metrics: `cosine`, `euclidean`, `dot_product`, `manhattan`.
+
+### 3. `schema_inspect` — Schema Exploration
+
+```json
+{
+  "name": "schema_inspect",
+  "arguments": {
+    "table": "users",
+    "tenant_id": "company-a"
+  }
+}
+```
+
+Returns tables, columns, types, primary keys, foreign keys, indexes, and RLS policies.
+
+## Claude Desktop Configuration
+
+```json
+{
+  "mcpServers": {
+    "baradb": {
+      "command": "/path/to/build/baramcp",
+      "args": ["--data-dir", "/path/to/data"]
+    }
+  }
+}
+```
+
+## Cursor Configuration
+
+```json
+{
+  "mcpServers": {
+    "baradb": {
+      "command": "/path/to/build/baramcp",
+      "args": ["--data-dir", "~/.baradb/data"]
+    }
+  }
+}
+```
+
+## Multi-Tenant Isolation
+
+Each MCP request can include `tenant_id` and `user_id`, set as session variables:
+- `app.tenant_id` — for RLS filtering
+- `app.user_id` — for `current_user` references
+
+RLS policies automatically filter data based on these variables.
+
+## JSON-RPC 2.0 Protocol
+
+The server uses JSON-RPC 2.0 over STDIO:
+
+```json
+// Request
+{"jsonrpc": "2.0", "id": 1, "method": "tools/call", "params": {...}}
+
+// Response
+{"jsonrpc": "2.0", "id": 1, "result": {"content": [{"type": "text", "text": "..."}]}}
+```