openspec: sms-metering-location-link change

openspec/changes/sms-metering-location-link/specs/sms-metering/spec.md

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+## ADDED Requirements
+
+### Requirement: Per-tag SMS credits
+Each tag SHALL track `sms_allocated`, `sms_used`, and `sms_period_start`. An allocation of 0 SHALL mean unmetered (no credit limit).
+
+#### Scenario: Allocated plan
+- **WHEN** an admin sets `sms_allocated = 10` on a tag
+- **THEN** the tag may send up to 10 SMS in the period
+
+#### Scenario: Unmetered default
+- **WHEN** a tag has `sms_allocated = 0`
+- **THEN** credit checks do not apply (transitional behaviour)
+
+### Requirement: Credits required to send
+Before sending an alert or finder-contact SMS on a metered tag, `sms_used < sms_allocated` SHALL hold. Each successful send SHALL increment `sms_used`. When exhausted, scans are recorded but no SMS is sent.
+
+#### Scenario: Send consumes a credit
+- **WHEN** an alert SMS is sent on a metered tag
+- **THEN** `sms_used` increments by one
+
+#### Scenario: Credits exhausted
+- **WHEN** `sms_used >= sms_allocated` on a metered tag
+- **THEN** further scans record but no SMS is sent