feat(01-01): compose file, env example, gitignore, bootstrap migration

- compose.yaml: postgres:16-alpine with pg_isready healthcheck (no seed mounts)
- .env.example: DATABASE_URL, PORT, ENV (D-15)
- .gitignore: bin/, tmp/, .env, generated tailwind.css, bootstrap-downloaded htmx.min.js, *_templ.go, sqlc output
- migrations/0001_init.sql: goose no-op bootstrap migration

backend/.env.example

@@ -0,0 +1,8 @@
+# Postgres connection string used by the web + worker binaries (and `just migrate`).
+DATABASE_URL=postgres://xtablo:xtablo@localhost:5432/xtablo?sslmode=disable
+
+# HTTP port for cmd/web.
+PORT=8080
+
+# Environment selector: "development" enables the slog text handler; "production" switches to JSON.
+ENV=development

backend/.gitignore

@@ -0,0 +1,24 @@
+# Local environment files (only .env.example is committed)
+.env
+.env.local
+
+# Bootstrap-downloaded CLIs and Tailwind binary (re-created by `just bootstrap`)
+bin/
+!bin/.gitkeep
+
+# air rebuild output
+tmp/
+
+# Tailwind-compiled CSS (re-created by `just generate` / `just styles-watch`)
+static/tailwind.css
+
+# HTMX bootstrap-downloaded by `just bootstrap` (never committed; the justfile is the single
+# authoritative source for the pinned HTMX version — see CONTEXT D-10)
+static/htmx.min.js
+
+# templ-generated Go files (regenerate via `templ generate` — Research Pitfall 1)
+*_templ.go
+
+# sqlc-generated code (regenerate via `sqlc generate`); keep the .gitkeep
+internal/db/sqlc/*.go
+!internal/db/sqlc/.gitkeep

backend/compose.yaml

@@ -0,0 +1,21 @@
+services:
+  postgres:
+    image: postgres:16-alpine
+    container_name: xtablo-backend-postgres
+    restart: unless-stopped
+    environment:
+      POSTGRES_DB: xtablo
+      POSTGRES_USER: xtablo
+      POSTGRES_PASSWORD: xtablo
+    ports:
+      - "5432:5432"
+    volumes:
+      - postgres_data:/var/lib/postgresql/data
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U xtablo -d xtablo"]
+      interval: 5s
+      timeout: 5s
+      retries: 10
+
+volumes:
+  postgres_data:

backend/migrations/0001_init.sql

@@ -0,0 +1,6 @@
+-- +goose Up
+-- Phase 1: no-op bootstrap migration. Real schema lands in Phase 2 (sessions/users) and Phase 3 (tablos).
+SELECT 1;
+
+-- +goose Down
+SELECT 1;