fix: correct sqlx migration directory path for Docker builds (Closes #3)

[ZacLou]

Closes #3

Changes

Fixes the sqlx::migrate!() compile-time path in both services so migrations are embedded correctly during Docker builds:

• API: ../../migrations → ../migrations
• Gateway: Same fix applied

Why

• ../../migrations from crates/api/src/main.rs resolves to migrations/ locally but can fail in Docker build contexts
• ../migrations uses the Cargo workspace manifest-relative path which is more reliable across build environments
• sqlx embeds migrations at compile time; the macro path must match the filesystem during cargo build