refactor: implement unified binary architecture

🏗️ **Major Architecture Refactoring: Separate CLI + Server → Unified Binary**

**Key Changes:**
✅ **Unified Binary**: Single 'insertr' binary with subcommands (enhance, serve)
✅ **Preserved Database Architecture**: Maintained sophisticated sqlc multi-DB setup
✅ **Smart Configuration**: Viper + YAML config with CLI flag precedence
✅ **Updated Build System**: Unified justfile, Air, and npm scripts

**Command Structure:**
- `insertr enhance [input-dir]` - Build-time content injection
- `insertr serve` - HTTP API server (dev + production modes)
- `insertr --config insertr.yaml` - YAML configuration support

**Architecture Benefits:**
- **Shared Database Layer**: Single source of truth for content models
- **Flexible Workflows**: Local DB for dev, remote API for production
- **Simple Deployment**: One binary for all use cases
- **Better UX**: Consistent configuration across build and runtime

**Preserved Features:**
- Multi-database support (SQLite + PostgreSQL)
- sqlc code generation and type safety
- Version control system with rollback
- Professional API endpoints
- Content enhancement pipeline

**Development Workflow:**
- `just dev` - Full-stack development (API server + demo site)
- `just serve` - API server only
- `just enhance` - Build-time content injection
- `air` - Hot reload unified binary

**Migration:** Consolidated insertr-cli/ and insertr-server/ → unified root structure

.air.toml

@@ -0,0 +1,46 @@
+root = "."
+testdata_dir = "testdata"
+tmp_dir = "tmp"
+
+[build]
+  args_bin = []
+  bin = "./tmp/insertr"
+  cmd = "go build -o ./tmp/insertr ."
+  delay = 1000
+  exclude_dir = ["tmp", "vendor", "testdata", "node_modules", "dist", "insertr-cli", "insertr-server"]
+  exclude_file = []
+  exclude_regex = ["_test.go"]
+  exclude_unchanged = false
+  follow_symlink = false
+  full_bin = "./tmp/insertr serve --dev-mode --db ./dev.db"
+  include_dir = ["cmd", "internal", "lib/src"]
+  include_ext = ["go", "tpl", "tmpl", "html", "js"]
+  include_file = []
+  kill_delay = "0s"
+  log = "build-errors.log"
+  poll = false
+  poll_interval = 0
+  post_cmd = []
+  pre_cmd = ["cd lib && npm run build"]
+  rerun = false
+  rerun_delay = 500
+  send_interrupt = false
+  stop_on_root = false
+
+[color]
+  app = ""
+  build = "yellow"
+  main = "magenta"
+  runner = "green"
+  watcher = "cyan"
+
+[log]
+  main_only = false
+  time = false
+
+[misc]
+  clean_on_exit = true
+
+[screen]
+  clear_on_rebuild = false
+  keep_scroll = true