chore: remove redundant migrations directory and migrate binary

All content from 003_add_device_codes.sql and 004_add_trigram_indexes.sql
was already absorbed into db/schema.sql (the single source of truth).

The migrations had additional problems:
- Broken numbering (started at 003, missing 001/002)
- 004 used CREATE INDEX CONCURRENTLY which fails inside sqlx transactions
- No production flow ever invoked the migrate binary

Removed: db/migrations/, src/bin/migrate.rs, migrations Cargo feature,
[[bin]] migrate target, and doc/database-migrations.md.

Cargo.toml

@@ -59,12 +59,6 @@ socket2 = { version = "0.6.2", features = ["all"] }
 [features]
 default = []
 test_utils = ["mockall"]
-migrations = ["sqlx/migrate"]
-
-[[bin]]
-name = "migrate"
-path = "src/bin/migrate.rs"
-required-features = ["migrations"]
 
 [profile.release]
 lto = "fat"