1525-07: Align Rust and database types

[josecelano]

Goal

Widen the download-counter type in Rust from u32 to u64 and widen the corresponding database columns from INTEGER (32-bit, MySQL) to BIGINT (64-bit), delivered as a versioned sqlx migration so the change is explicit, testable, and tracked as a forward schema change.

Spec

docs/issues/1525-07-align-rust-and-db-types.md

Parent issue

Part of the persistence overhaul EPIC: #1525

Tasks

• Task 1 — Add migration files (20260409120000_torrust_tracker_widen_download_counters.sql for both sqlite/ and mysql/)
• Task 2 — Widen NumberOfDownloads from u32 → u64, update SwarmMetadata.downloaded, add decode_counter/encode_counter helpers to both drivers, fix all cascade compilation errors
• Task 3 — Add tests: MySQL column widening round-trip, SQLite no-op, encode_counter error on values above i64::MAX

Acceptance Criteria

• packages/tracker-core/migrations/sqlite/20260409120000_torrust_tracker_widen_download_counters.sql exists and is a comment-only no-op.
• packages/tracker-core/migrations/mysql/20260409120000_torrust_tracker_widen_download_counters.sql exists and widens torrents.completed and torrent_aggregate_metrics.value to BIGINT.
• NumberOfDownloads = u64 in packages/primitives/src/lib.rs.
• SwarmMetadata.downloaded uses NumberOfDownloads; bare u32 is removed from that field.
• Both driver files use explicit decode_counter / encode_counter helpers for all counter-column reads and writes.
• encode_counter returns an error (not a panic, not silent truncation) for values above i64::MAX.
• A test verifies round-trip of a value larger than u32::MAX for each backend.
• A test verifies the encode error path for values above i64::MAX.
• No as u32 casts or compiler-suppression attributes introduced by this subissue.
• cargo test --workspace --all-targets passes.
• linter all exits with code 0.

References

• EPIC: Overhaul persistence #1525
• Prerequisite: 1525-06: Introduce schema migrations #1719 (1525-06: Introduce schema migrations — must be completed first)
• Next: 1525-08 (Add PostgreSQL driver support)