feat: add feedback resolver

migrations/0017_add_feedback_resolved_at.down.sql

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+DROP INDEX IF EXISTS idx_feedback_unresolved;
+ALTER TABLE feedback DROP COLUMN IF EXISTS resolved_at;

migrations/0017_add_feedback_resolved_at.up.sql

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+-- Track when a feedback row was resolved.
+--
+-- NULL = still open. Non-NULL = closed at that moment. The action is
+-- one-way: the resolve endpoint enforces `WHERE resolved_at IS NULL` so
+-- a row already marked stays marked, and a subsequent re-resolve is a
+-- no-op (no new email goes out).
+--
+-- We don't store "who" resolved it because the /stats page surfaces
+-- this to the team and any audit-quality "who closed what" trail would
+-- want its own table — out of scope for now.
+ALTER TABLE feedback
+    ADD COLUMN IF NOT EXISTS resolved_at TIMESTAMPTZ;
+
+-- Partial index makes "give me the still-open feedback" cheap as the
+-- table grows. The /stats page lists newest-first regardless of state,
+-- so the existing created_at index still serves its read path.
+CREATE INDEX IF NOT EXISTS idx_feedback_unresolved
+    ON feedback (created_at DESC)
+    WHERE resolved_at IS NULL;