feat: add Nginx reverse proxy for robust CORS handling

Replace direct host port mapping with an Nginx reverse proxy that properly
handles CORS for any origin, including Obsidian's app://obsidian.md.

- docker-compose.yml: Add nginx service on port 8666, route to internal
  chroma:8000. Remove CORS env var from ChromaDB (handled by proxy now).

- nginx.conf (new): Minimal alpine config with permissive CORS headers,
  preflight OPTIONS handling, and streaming support (proxy_buffering off).

This fixes cross-origin access from remote Obsidian clients connecting over
Tailscale or VPN.

docker-compose.yml

@@ -1,15 +1,24 @@
 services:
   chroma:
     image: chromadb/chroma:latest
-    ports:
+    expose:
-      - '8666:8000'
+      - '8000'
     environment:
       - CHROMA_SERVER_HOST=0.0.0.0
       - CHROMA_SERVER_HTTP_PORT=8000
-      - CHROMA_SERVER_CORS_ALLOW_ORIGINS=["*"]
     volumes:
       - chroma_data:/chroma/chroma
     restart: unless-stopped
 
+  nginx:
+    image: nginx:alpine
+    ports:
+      - '8666:8666'
+    volumes:
+      - ./nginx.conf:/etc/nginx/nginx.conf:ro
+    depends_on:
+      - chroma
+    restart: unless-stopped
+
 volumes:
   chroma_data:

nginx.conf

@@ -0,0 +1,41 @@
+events {
+    worker_connections 1024;
+}
+
+http {
+    server {
+        listen 8666;
+        server_name localhost;
+
+        # CORS: allow any origin, including app://obsidian.md
+        add_header 'Access-Control-Allow-Origin' '*' always;
+        add_header 'Access-Control-Allow-Methods' 'GET, POST, PUT, DELETE, OPTIONS' always;
+        add_header 'Access-Control-Allow-Headers' 'DNT,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,Range,Authorization' always;
+        add_header 'Access-Control-Expose-Headers' 'Content-Length,Content-Range' always;
+
+        # Handle preflight OPTIONS
+        if ($request_method = 'OPTIONS') {
+            add_header 'Access-Control-Allow-Origin' '*' always;
+            add_header 'Access-Control-Allow-Methods' 'GET, POST, PUT, DELETE, OPTIONS' always;
+            add_header 'Access-Control-Allow-Headers' 'DNT,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,Range,Authorization' always;
+            add_header 'Access-Control-Max-Age' 1728000;
+            add_header 'Content-Type' 'text/plain; charset=utf-8';
+            add_header 'Content-Length' 0;
+            return 204;
+        }
+
+        location / {
+            proxy_pass http://chroma:8000;
+            proxy_http_version 1.1;
+            proxy_set_header Host $host;
+            proxy_set_header X-Real-IP $remote_addr;
+            proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+            proxy_set_header X-Forwarded-Proto $scheme;
+
+            # Support streaming
+            proxy_buffering off;
+            proxy_cache off;
+            proxy_read_timeout 86400s;
+        }
+    }
+}