chore: add Docker build volume and configure OpenCode

Add a named volume for backend build artifacts to prevent root-owned files
created inside the container from blocking host Gradle builds. This follows
the same pattern as the existing backend-gradle-project volume.

Configure OpenCode with LSP, formatter, auto-compaction, and file watcher
settings for improved development experience.

Changes:
- docker-compose.yml: add backend-build:/app/backend/build volume
- opencode.json: enable lsp, formatter, auto-compaction, prune, and
  file watcher with ignore patterns for node_modules, .git, dist, build

docker-compose.yml

@@ -38,6 +38,7 @@ services:
     volumes:
       - .:/app
       - backend-gradle-project:/app/.gradle
+      - backend-build:/app/backend/build
       - gradle-cache:/root/.gradle
 
   frontend:
@@ -58,3 +59,4 @@ volumes:
   pgdata:
   gradle-cache:
   backend-gradle-project:
+  backend-build:

opencode.json

@@ -1,6 +1,8 @@
 {
   "$schema": "https://opencode.ai/config.json",
   "instructions": ["CODING_GUIDELINES.md", "REQUIREMENTS.md"],
+  "lsp": true,
+  "formatter": true,
   "permission": {
     "edit": "ask",
     "bash": "ask"
@@ -8,5 +10,12 @@
   "tools": {
     "websearch": true,
     "codesearch": true
+  },
+  "compaction": {
+    "auto": true,
+    "prune": true
+  },
+  "watcher": {
+    "ignore": ["node_modules/**", ".git/**", "dist/**", "build/**", "**/target/**"]
   }
 }