Add database support with fluent query builder

[kacy]

Summary

• Implements PostgREST database operations (CRUD) in the previously empty src/db.rs file
• Adds a fluent QueryBuilder API for constructing and executing database queries
• Exports the db module publicly and adds from() method to Supabase struct

Features

• CRUD Operations: select, insert, update, delete
• Filter Methods: eq, neq, gt, gte, lt, lte, like, ilike, in_, is_null, not_null
• Query Modifiers: order, limit, offset
• Fluent API: Chain methods for readable query construction

Usage Examples

// Select with filters
client.from("users")
    .select("id,name,email")
    .eq("status", "active")
    .order("created_at.desc")
    .limit(10)
    .execute()
    .await?;

// Insert
client.from("users").insert(&user_data).execute().await?;

// Update
client.from("users").update(&updates).eq("id", "123").execute().await?;

// Delete
client.from("users").delete().eq("id", "123").execute().await?;