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/sqlite/setup.sql.go

@@ -0,0 +1,45 @@
+// Code generated by sqlc. DO NOT EDIT.
+// versions:
+//   sqlc v1.29.0
+// source: setup.sql
+
+package sqlite
+
+import (
+	"context"
+)
+
+const initializeSchema = `-- name: InitializeSchema :exec
+CREATE TABLE IF NOT EXISTS content (
+    id TEXT NOT NULL,
+    site_id TEXT NOT NULL,
+    value TEXT NOT NULL,
+    type TEXT NOT NULL CHECK (type IN ('text', 'markdown', 'link')),
+    created_at INTEGER DEFAULT (strftime('%s', 'now')) NOT NULL,
+    updated_at INTEGER DEFAULT (strftime('%s', 'now')) NOT NULL,
+    last_edited_by TEXT DEFAULT 'system' NOT NULL,
+    PRIMARY KEY (id, site_id)
+)
+`
+
+func (q *Queries) InitializeSchema(ctx context.Context) error {
+	_, err := q.db.ExecContext(ctx, initializeSchema)
+	return err
+}
+
+const initializeVersionsTable = `-- name: InitializeVersionsTable :exec
+CREATE TABLE IF NOT EXISTS content_versions (
+    version_id INTEGER PRIMARY KEY AUTOINCREMENT,
+    content_id TEXT NOT NULL,
+    site_id TEXT NOT NULL,
+    value TEXT NOT NULL,
+    type TEXT NOT NULL,
+    created_at INTEGER DEFAULT (strftime('%s', 'now')) NOT NULL,
+    created_by TEXT DEFAULT 'system' NOT NULL
+)
+`
+
+func (q *Queries) InitializeVersionsTable(ctx context.Context) error {
+	_, err := q.db.ExecContext(ctx, initializeVersionsTable)
+	return err
+}