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/postgresql/schema.sql

@@ -0,0 +1,42 @@
+-- PostgreSQL-specific schema with BIGINT UNIX timestamps
+-- Main content table (current versions only)
+CREATE TABLE 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 BIGINT DEFAULT EXTRACT(EPOCH FROM NOW()) NOT NULL,
+    updated_at BIGINT DEFAULT EXTRACT(EPOCH FROM NOW()) NOT NULL,
+    last_edited_by TEXT DEFAULT 'system' NOT NULL,
+    PRIMARY KEY (id, site_id)
+);
+
+-- Version history table for rollback functionality
+CREATE TABLE content_versions (
+    version_id SERIAL PRIMARY KEY,
+    content_id TEXT NOT NULL,
+    site_id TEXT NOT NULL,
+    value TEXT NOT NULL,
+    type TEXT NOT NULL,
+    created_at BIGINT DEFAULT EXTRACT(EPOCH FROM NOW()) NOT NULL,
+    created_by TEXT DEFAULT 'system' NOT NULL
+);
+
+-- Indexes for performance
+CREATE INDEX IF NOT EXISTS idx_content_site_id ON content(site_id);
+CREATE INDEX IF NOT EXISTS idx_content_updated_at ON content(updated_at);
+CREATE INDEX IF NOT EXISTS idx_content_versions_lookup ON content_versions(content_id, site_id, created_at DESC);
+
+-- Function and trigger to automatically update updated_at timestamp
+CREATE OR REPLACE FUNCTION update_updated_at_column()
+RETURNS TRIGGER AS $$
+BEGIN
+    NEW.updated_at = EXTRACT(EPOCH FROM NOW());
+    RETURN NEW;
+END;
+$$ language 'plpgsql';
+
+CREATE TRIGGER update_content_updated_at
+    BEFORE UPDATE ON content
+    FOR EACH ROW
+    EXECUTE FUNCTION update_updated_at_column();