bug: safely enforce MacTrack site-name uniqueness

[somethingwithproof]

Problem

mac_track_sites has only a primary key on site_id, so duplicate site names—including duplicate Default rows—are structurally possible. The Default-site repair in #357 uses a database-scoped advisory lock and a conditional insert, which is safe on a single database server but is not a substitute for a unique constraint in proxied or multi-primary deployments.

Adding a unique index directly in the 1.2 release fix is unsafe because existing installations may already contain duplicate custom or Default names and their references must be reconciled deliberately.

Required validation

• Reproduce upgrades containing duplicate Default and duplicate custom site names.
• Define deterministic merge/rename behavior without losing device relationships or operator-authored metadata.
• Add a guarded, restartable uniqueness migration for MariaDB and MySQL.
• Verify new installs and upgrades end with an equivalent unique constraint.
• Exercise concurrent creation without relying on connection-scoped advisory locks.
• Treat duplicate-key outcomes as a satisfied postcondition when another writer wins.

Relationships

Found during review of #357. Coordinate with the broader schema work in #104 and the legacy migration defect in #359.

[somethingwithproof]

Update from #357 review: the Default-site creation path now uses INSERT IGNORE with fixed site_id = 1, and its Docker race assigns distinct advisory-lock namespaces to all eight workers. That removes advisory-lock dependence for the Default-row invariant on both MariaDB and MySQL.

This issue remains open for the broader migration problem: safely reconciling and preventing duplicate custom site names without losing device relationships or operator-authored metadata.

[somethingwithproof]

Further #357 review found that fixing Default to site_id = 1 is unsafe: site deletion currently leaves mac_track_arp and mac_track_dot1x rows behind, so reusing ID 1 could misattribute orphaned history to the new Default site. That approach was reverted.

#357 retains the single-primary advisory-lock improvement and now fails CLI installs non-zero plus retries from the poller. This issue remains the place to design topology-independent uniqueness together with legacy duplicate reconciliation and orphan cleanup.