feat: modernize build tooling and add CI/CD workflow

Switch to Bun for 2-5x faster frontend builds, implement cargo-chef for
reliable Rust dependency caching, and add Biome for fast code
formatting.

Build system improvements:
- Replace pnpm with Bun for frontend package management
- Add cargo-chef to Dockerfile for better Rust build layer caching
- Update all commands to use bun instead of pnpm

Developer experience:
- Add comprehensive Justfile commands (format, format-check, db)
- Implement automated PostgreSQL Docker setup with random port
allocation
- Add stricter checks (--deny warnings on clippy, --all-features flag)

Code quality:
- Add Biome formatter for 10-100x faster TypeScript/JavaScript
formatting
- Add GitHub Actions CI/CD workflow for automated checks
- Update .dockerignore with comprehensive exclusions
- Format all code with cargo fmt (Rust) and Biome (TypeScript)

All changes maintain backward compatibility and can be tested
incrementally.

Justfile

@@ -3,19 +3,63 @@ default_services := "bot,web,scraper"
 default:
     just --list
 
+# Run all checks (format, clippy, tests, lint)
 check:
-    cargo check
-    cargo clippy
+    cargo fmt --all -- --check
+    cargo clippy --all-features -- --deny warnings
     cargo nextest run
-    pnpm run -C web lint
+    bun run --cwd web typecheck
+    bun run --cwd web lint
+
+# Format all Rust and TypeScript code
+format:
+    cargo fmt --all
+    bun run --cwd web format
+
+# Check formatting without modifying (CI-friendly)
+format-check:
+    cargo fmt --all -- --check
+    bun run --cwd web format:check
+
+# Start PostgreSQL in Docker and update .env with connection string
+db:
+    #!/usr/bin/env bash
+    set -euo pipefail
+
+    # Find available port
+    PORT=$(shuf -i 49152-65535 -n 1)
+    while ss -tlnp 2>/dev/null | grep -q ":$PORT "; do
+        PORT=$(shuf -i 49152-65535 -n 1)
+    done
+
+    # Start PostgreSQL container
+    docker run -d \
+        --name banner-postgres \
+        -e POSTGRES_PASSWORD=banner \
+        -e POSTGRES_USER=banner \
+        -e POSTGRES_DB=banner \
+        -p "$PORT:5432" \
+        postgres:17-alpine
+
+    # Update .env file
+    DB_URL="postgresql://banner:banner@localhost:$PORT/banner"
+    if [ -f .env ]; then
+        sed -i.bak "s|^DATABASE_URL=.*|DATABASE_URL=$DB_URL|" .env
+    else
+        echo "DATABASE_URL=$DB_URL" > .env
+    fi
+
+    echo "PostgreSQL started on port $PORT"
+    echo "DATABASE_URL=$DB_URL"
+    echo "Run: sqlx migrate run"
 
 # Auto-reloading frontend server
 frontend:
-    pnpm run -C web dev
+    bun run --cwd web dev
 
 # Production build of frontend
 build-frontend:
-    pnpm run -C web build
+    bun run --cwd web build
 
 # Auto-reloading backend server
 backend *ARGS:
@@ -23,15 +67,13 @@ backend *ARGS:
 
 # Production build
 build:
-    pnpm run -C web build
+    bun run --cwd web build
     cargo build --release --bin banner
 
-# Run auto-reloading development build with release characteristics (frontend is embedded, non-auto-reloading)
-# This is useful for testing backend release-mode details.
+# Run auto-reloading development build with release characteristics
 dev-build *ARGS='--services web --tracing pretty': build-frontend
     bacon --headless run -- --profile dev-release -- {{ARGS}}
 
 # Auto-reloading development build for both frontend and backend
-# Will not notice if either the frontend/backend crashes, but will generally be resistant to stopping on their own.
 [parallel]
 dev *ARGS='--services web,bot': frontend (backend ARGS)