feat: add SQL migration system using sqlx::migrate!()

Replace manual schema.sql application with sqlx's built-in migration
system. Migrations are embedded at compile time and tracked in the
_sqlx_migrations table. Pending migrations run automatically on startup.

- Move db/schema.sql → migrations/20260307000000_initial_schema.sql
- Remove apply_schema() and split_sql_statements() from db.rs
- Add run_migrations() using sqlx::migrate!() macro
- Remove docker-compose schema.sql mount (app handles it now)
- Enable sqlx "migrate" feature in Cargo.toml

Future schema changes: add a new timestamped .sql in migrations/.

Closes #190

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Cargo.toml

@@ -28,7 +28,7 @@ mime_guess = "2.0.5"
 uuid = { version = "1.21.0", features = ["v4", "serde"] }
 thiserror = "2.0.18"
 mockall = { version = "0.14.0", optional = true }
-sqlx = { version = "0.8.6", features = ["postgres", "runtime-tokio", "tls-rustls", "chrono", "uuid", "json"] }
+sqlx = { version = "0.8.6", features = ["postgres", "runtime-tokio", "tls-rustls", "chrono", "uuid", "json", "migrate"] }
 jsonwebtoken = { version = "10.3.0", features = ["rust_crypto"] }
 argon2 = "0.5.3"
 rand_core = { version = "0.6", features = ["std", "getrandom"] }