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

sqlc.yaml

@@ -0,0 +1,31 @@
+version: "2"
+sql:
+  # SQLite configuration for development
+  - name: "sqlite"
+    engine: "sqlite"
+    queries: ["db/queries/", "db/sqlite/setup.sql"]
+    schema: "db/sqlite/schema.sql"
+    gen:
+      go:
+        package: "sqlite"
+        out: "internal/db/sqlite"
+        emit_json_tags: true
+        emit_prepared_queries: false
+        emit_interface: true
+        emit_exact_table_names: false
+        emit_pointers_for_null_types: false  # All fields are NOT NULL now
+        
+  # PostgreSQL configuration for production
+  - name: "postgresql"
+    engine: "postgresql"
+    queries: ["db/queries/", "db/postgresql/setup.sql"]
+    schema: "db/postgresql/schema.sql"
+    gen:
+      go:
+        package: "postgresql" 
+        out: "internal/db/postgresql"
+        emit_json_tags: true
+        emit_prepared_queries: false
+        emit_interface: true
+        emit_exact_table_names: false
+        emit_pointers_for_null_types: false  # All fields are NOT NULL now