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/db/postgresql/querier.go

@@ -0,0 +1,31 @@
+// Code generated by sqlc. DO NOT EDIT.
+// versions:
+//   sqlc v1.29.0
+
+package postgresql
+
+import (
+	"context"
+)
+
+type Querier interface {
+	CreateContent(ctx context.Context, arg CreateContentParams) (Content, error)
+	CreateContentSiteIndex(ctx context.Context) error
+	CreateContentUpdatedAtIndex(ctx context.Context) error
+	CreateContentVersion(ctx context.Context, arg CreateContentVersionParams) error
+	CreateUpdateFunction(ctx context.Context) error
+	CreateVersionsLookupIndex(ctx context.Context) error
+	DeleteContent(ctx context.Context, arg DeleteContentParams) error
+	DeleteOldVersions(ctx context.Context, arg DeleteOldVersionsParams) error
+	GetAllContent(ctx context.Context, siteID string) ([]Content, error)
+	GetAllVersionsForSite(ctx context.Context, arg GetAllVersionsForSiteParams) ([]GetAllVersionsForSiteRow, error)
+	GetBulkContent(ctx context.Context, arg GetBulkContentParams) ([]Content, error)
+	GetContent(ctx context.Context, arg GetContentParams) (Content, error)
+	GetContentVersion(ctx context.Context, versionID int32) (ContentVersion, error)
+	GetContentVersionHistory(ctx context.Context, arg GetContentVersionHistoryParams) ([]ContentVersion, error)
+	InitializeSchema(ctx context.Context) error
+	InitializeVersionsTable(ctx context.Context) error
+	UpdateContent(ctx context.Context, arg UpdateContentParams) (Content, error)
+}
+
+var _ Querier = (*Queries)(nil)