feat: add feedback acknowledgement

migrations/0018_add_feedback_acknowledged_at.down.sql

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+ALTER TABLE feedback DROP COLUMN IF EXISTS acknowledged_at;

migrations/0018_add_feedback_acknowledged_at.up.sql

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+-- Track when a feedback row was acknowledged.
+--
+-- Acknowledgement is the intermediate state between "open" and
+-- "resolved": the team has seen the feedback and (if it needs work) the
+-- devs have started on it. The author gets an email saying so, with a
+-- promise of a second email once it's actually fixed/implemented.
+--
+-- NULL = not yet acknowledged. Non-NULL = acknowledged at that moment.
+-- Like resolved_at, the action is one-way and idempotent: the endpoint
+-- enforces `WHERE acknowledged_at IS NULL` so the acknowledgement email
+-- fires at most once.
+--
+-- The two columns are independent stamps. An admin may acknowledge then
+-- later resolve (two emails), or resolve directly without acknowledging
+-- (one email). Resolving does not require a prior acknowledgement.
+ALTER TABLE feedback
+    ADD COLUMN IF NOT EXISTS acknowledged_at TIMESTAMPTZ;