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

insertr.yaml

@@ -0,0 +1,25 @@
+# Insertr Configuration File
+# This file provides default configuration for the unified insertr binary
+
+# Database configuration
+database:
+  path: "./insertr.db"  # SQLite file path or PostgreSQL connection string
+
+# API configuration (for remote content API)
+api:
+  url: ""               # Content API URL (leave empty to use local database)
+  key: ""               # API authentication key
+
+# Server configuration
+server:
+  port: 8080            # HTTP server port
+  dev_mode: false       # Enable development mode features
+
+# Build configuration
+build:
+  input: "./src"        # Default input directory for enhancement
+  output: "./dist"      # Default output directory for enhanced files
+
+# Global settings
+site_id: "demo"         # Default site ID for content lookup
+mock_content: false     # Use mock content instead of real data