Add hot-reload docker-compose.dev.yml

Dev stack with live reload: Postgres + backend (hypercorn --reload, src mounted)
+ Vite dev server (mounted, proxies /api to the api service). vite proxy target
is now env-configurable (VITE_API_TARGET, defaults localhost:5000 for host dev).
README documents both compose files (dev hot-reload vs prod image).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

docker-compose.dev.yml

@@ -0,0 +1,67 @@
+# Hot-reload DEV stack: Postgres + backend (hypercorn --reload) + Vite dev server.
+# Edit code on the host and changes reload live. Per family rule 12 the agent does
+# NOT start this — run it yourself:
+#
+#   docker compose -f docker-compose.dev.yml up
+#
+# Then open http://localhost:5173 (the Vite dev server proxies /api to the backend).
+# (docker-compose.yml, by contrast, builds + runs the production image on :5000.)
+services:
+  db:
+    image: postgres:16-alpine
+    environment:
+      POSTGRES_USER: thoughtsync
+      POSTGRES_PASSWORD: thoughtsync
+      POSTGRES_DB: thoughtsync
+    volumes:
+      - thoughtsync-dev-db:/var/lib/postgresql/data
+    ports:
+      - "5432:5432"
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U thoughtsync"]
+      interval: 5s
+      timeout: 5s
+      retries: 10
+
+  api:
+    image: python:3.12-slim
+    working_dir: /app
+    depends_on:
+      db:
+        condition: service_healthy
+    environment:
+      THOUGHTSYNC_DATABASE_URL: postgresql+asyncpg://thoughtsync:thoughtsync@db:5432/thoughtsync
+      THOUGHTSYNC_DATA_DIR: /data
+      PYTHONPATH: /app/src
+    volumes:
+      - ./pyproject.toml:/app/pyproject.toml
+      - ./alembic.ini:/app/alembic.ini
+      - ./alembic:/app/alembic
+      - ./src:/app/src
+      - thoughtsync-dev-data:/data
+    ports:
+      - "5000:5000"
+    # Install deps, run migrations, then serve with live reload on the mounted src.
+    command: >
+      sh -c "pip install --quiet -e . &&
+             alembic upgrade head &&
+             hypercorn 'thoughtsync.app:create_app()' --bind 0.0.0.0:5000 --reload"
+
+  web:
+    image: node:22-alpine
+    working_dir: /app
+    depends_on:
+      - api
+    environment:
+      # Vite proxies /api to the backend service (see frontend/vite.config.ts).
+      VITE_API_TARGET: http://api:5000
+    volumes:
+      - ./frontend:/app
+      - /app/node_modules
+    ports:
+      - "5173:5173"
+    command: sh -c "npm install && npm run dev -- --host 0.0.0.0"
+
+volumes:
+  thoughtsync-dev-db:
+  thoughtsync-dev-data: