Add database integration

[arferreira]

Summary

First-class database integration for Rapina with SeaORM. Discovered and implemented through dogfooding with a real blog API.

Features:

• DatabaseConfig - environment-aware configuration (DATABASE_URL, pool settings)
• Db extractor - clean access to database connection in handlers
• DbError wrapper with IntoApiError - reduces error mapping boilerplate
• .with_database(config).await? - easy setup on Rapina builder
• Feature flags: postgres, mysql, sqlite
• Re-exports http, hyper, schemars, sea_orm so users don't need them in Cargo.toml

Usage:

use rapina::prelude::*;
use rapina::database::{DatabaseConfig, Db, DbError};
use rapina::sea_orm;
use sea_orm::{EntityTrait, ActiveModelTrait};

#[get("/posts")]
async fn list_posts(db: Db) -> Result<Json<Vec<Post>>> {
    let posts = Post::find().all(db.conn()).await.map_err(DbError::from)?;
    Ok(Json(posts))
}

#[tokio::main]
async fn main() -> std::io::Result<()> {
    let db_config = DatabaseConfig::from_env()?;

    Rapina::new()
        .with_database(db_config).await?
        .router(router)
        .listen("127.0.0.1:3000").await
}

Environment variables:

• DATABASE_URL (required)
• DATABASE_MAX_CONNECTIONS (default: 10)
• DATABASE_MIN_CONNECTIONS (default: 1)
• DATABASE_CONNECT_TIMEOUT (default: 30s)
• DATABASE_IDLE_TIMEOUT (default: 600s)

Also includes macro fixes from #141 (cherry-picked).

Closes #86

Checklist

• cargo fmt passes
• cargo clippy has no warnings
• Tests pass
• Documentation updated (if needed)

[uemuradevexe]

LGTM