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

db/queries/content.sql

@@ -0,0 +1,30 @@
+-- name: GetContent :one
+SELECT id, site_id, value, type, created_at, updated_at, last_edited_by
+FROM content 
+WHERE id = sqlc.arg(id) AND site_id = sqlc.arg(site_id);
+
+-- name: GetAllContent :many
+SELECT id, site_id, value, type, created_at, updated_at, last_edited_by
+FROM content 
+WHERE site_id = sqlc.arg(site_id)
+ORDER BY updated_at DESC;
+
+-- name: GetBulkContent :many
+SELECT id, site_id, value, type, created_at, updated_at, last_edited_by
+FROM content 
+WHERE site_id = sqlc.arg(site_id) AND id IN (sqlc.slice('ids'));
+
+-- name: CreateContent :one
+INSERT INTO content (id, site_id, value, type, last_edited_by)
+VALUES (sqlc.arg(id), sqlc.arg(site_id), sqlc.arg(value), sqlc.arg(type), sqlc.arg(last_edited_by))
+RETURNING id, site_id, value, type, created_at, updated_at, last_edited_by;
+
+-- name: UpdateContent :one
+UPDATE content 
+SET value = sqlc.arg(value), type = sqlc.arg(type), last_edited_by = sqlc.arg(last_edited_by)
+WHERE id = sqlc.arg(id) AND site_id = sqlc.arg(site_id)
+RETURNING id, site_id, value, type, created_at, updated_at, last_edited_by;
+
+-- name: DeleteContent :exec
+DELETE FROM content 
+WHERE id = sqlc.arg(id) AND site_id = sqlc.arg(site_id);

db/queries/versions.sql

@@ -0,0 +1,29 @@
+-- name: CreateContentVersion :exec
+INSERT INTO content_versions (content_id, site_id, value, type, created_by)
+VALUES (sqlc.arg(content_id), sqlc.arg(site_id), sqlc.arg(value), sqlc.arg(type), sqlc.arg(created_by));
+
+-- name: GetContentVersionHistory :many
+SELECT version_id, content_id, site_id, value, type, created_at, created_by
+FROM content_versions
+WHERE content_id = sqlc.arg(content_id) AND site_id = sqlc.arg(site_id)
+ORDER BY created_at DESC
+LIMIT sqlc.arg(limit_count);
+
+-- name: GetContentVersion :one
+SELECT version_id, content_id, site_id, value, type, created_at, created_by
+FROM content_versions
+WHERE version_id = sqlc.arg(version_id);
+
+-- name: GetAllVersionsForSite :many
+SELECT 
+    cv.version_id, cv.content_id, cv.site_id, cv.value, cv.type, cv.created_at, cv.created_by,
+    c.value as current_value
+FROM content_versions cv
+LEFT JOIN content c ON cv.content_id = c.id AND cv.site_id = c.site_id
+WHERE cv.site_id = sqlc.arg(site_id)
+ORDER BY cv.created_at DESC
+LIMIT sqlc.arg(limit_count);
+
+-- name: DeleteOldVersions :exec
+DELETE FROM content_versions 
+WHERE created_at < sqlc.arg(created_before) AND site_id = sqlc.arg(site_id);