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

internal/api/models.go

@@ -0,0 +1,52 @@
+package api
+
+import "time"
+
+// API request/response models
+type ContentItem struct {
+	ID           string    `json:"id"`
+	SiteID       string    `json:"site_id"`
+	Value        string    `json:"value"`
+	Type         string    `json:"type"`
+	CreatedAt    time.Time `json:"created_at"`
+	UpdatedAt    time.Time `json:"updated_at"`
+	LastEditedBy string    `json:"last_edited_by"`
+}
+
+type ContentVersion struct {
+	VersionID int64     `json:"version_id"`
+	ContentID string    `json:"content_id"`
+	SiteID    string    `json:"site_id"`
+	Value     string    `json:"value"`
+	Type      string    `json:"type"`
+	CreatedAt time.Time `json:"created_at"`
+	CreatedBy string    `json:"created_by"`
+}
+
+type ContentResponse struct {
+	Content []ContentItem `json:"content"`
+}
+
+type ContentVersionsResponse struct {
+	Versions []ContentVersion `json:"versions"`
+}
+
+// Request models
+type CreateContentRequest struct {
+	ID        string `json:"id"`
+	SiteID    string `json:"site_id,omitempty"`
+	Value     string `json:"value"`
+	Type      string `json:"type"`
+	CreatedBy string `json:"created_by,omitempty"`
+}
+
+type UpdateContentRequest struct {
+	Value     string `json:"value"`
+	Type      string `json:"type,omitempty"`
+	UpdatedBy string `json:"updated_by,omitempty"`
+}
+
+type RollbackContentRequest struct {
+	VersionID    int64  `json:"version_id"`
+	RolledBackBy string `json:"rolled_back_by,omitempty"`
+}