feat(photos): add square medium variant for sharp grid thumbnails

backend/migrations/000011_photo_medium_variant.down.sql

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+-- Drop medium bindings before narrowing the role constraint again.
+DELETE FROM photo_objects WHERE role = 'medium';
+ALTER TABLE photo_objects DROP CONSTRAINT IF EXISTS photo_objects_role_check;
+ALTER TABLE photo_objects ADD CONSTRAINT photo_objects_role_check
+    CHECK (role IN ('original', 'thumb', 'preview'));

backend/migrations/000011_photo_medium_variant.up.sql

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+-- The grid serves a square medium variant between thumb and preview.
+ALTER TABLE photo_objects DROP CONSTRAINT IF EXISTS photo_objects_role_check;
+ALTER TABLE photo_objects ADD CONSTRAINT photo_objects_role_check
+    CHECK (role IN ('original', 'thumb', 'medium', 'preview'));
+
+-- Regenerate variants for existing photos so they gain the medium variant and a
+-- square thumb. The worker rebinds roles; superseded variants are reclaimed by
+-- the reference-counting collector.
+INSERT INTO variant_jobs (photo_id)
+SELECT id FROM photos
+ON CONFLICT (photo_id) DO NOTHING;

backend/migrations/sqlite/000003_photo_medium_variant.down.sql

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+-- Drop medium bindings and restore the original three-role constraint.
+DELETE FROM photo_objects WHERE role = 'medium';
+
+CREATE TABLE photo_objects_old (
+    photo_id  TEXT NOT NULL REFERENCES photos(id) ON DELETE CASCADE,
+    object_id TEXT NOT NULL REFERENCES objects(id) ON DELETE RESTRICT,
+    role      TEXT NOT NULL CHECK (role IN ('original', 'thumb', 'preview')),
+    PRIMARY KEY (photo_id, role)
+);
+
+INSERT INTO photo_objects_old (photo_id, object_id, role)
+SELECT photo_id, object_id, role FROM photo_objects;
+
+DROP TABLE photo_objects;
+
+ALTER TABLE photo_objects_old RENAME TO photo_objects;
+
+CREATE INDEX IF NOT EXISTS photo_objects_object_idx ON photo_objects (object_id);

backend/migrations/sqlite/000003_photo_medium_variant.up.sql

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+-- SQLite cannot alter a CHECK constraint, so recreate photo_objects with the
+-- extended role set (medium added) and copy the existing bindings across.
+CREATE TABLE photo_objects_new (
+    photo_id  TEXT NOT NULL REFERENCES photos(id) ON DELETE CASCADE,
+    object_id TEXT NOT NULL REFERENCES objects(id) ON DELETE RESTRICT,
+    role      TEXT NOT NULL CHECK (role IN ('original', 'thumb', 'medium', 'preview')),
+    PRIMARY KEY (photo_id, role)
+);
+
+INSERT INTO photo_objects_new (photo_id, object_id, role)
+SELECT photo_id, object_id, role FROM photo_objects;
+
+DROP TABLE photo_objects;
+
+ALTER TABLE photo_objects_new RENAME TO photo_objects;
+
+CREATE INDEX IF NOT EXISTS photo_objects_object_idx ON photo_objects (object_id);
+
+-- Regenerate variants for existing photos so they gain the medium variant and a
+-- square thumb.
+INSERT INTO variant_jobs (photo_id)
+SELECT id FROM photos
+WHERE 1
+ON CONFLICT (photo_id) DO NOTHING;