fix: allow re-allocation of released subnets by removing global cidr unique constraint

migrations/20241001140000_allow_released_cidr.sql

@@ -0,0 +1,34 @@
+-- Remove the global UNIQUE constraint on allocations.cidr so that a released
+-- subnet can be re-allocated later. Enforce uniqueness only among active and
+-- reserved allocations within the same pool.
+
+CREATE TABLE IF NOT EXISTS allocations_new (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    pool_id INTEGER NOT NULL REFERENCES pools(id),
+    cidr TEXT NOT NULL,
+    prefix_length INTEGER NOT NULL,
+    address_count INTEGER NOT NULL,
+    usable_host_count INTEGER NOT NULL,
+    note TEXT NOT NULL,
+    status TEXT NOT NULL CHECK(status IN ('active', 'reserved', 'released')),
+    created_at TEXT NOT NULL,
+    created_by TEXT NOT NULL DEFAULT 'local',
+    released_at TEXT NULL,
+    released_by TEXT NULL,
+    release_note TEXT NULL
+);
+
+INSERT INTO allocations_new SELECT * FROM allocations;
+
+DROP TABLE allocations;
+
+ALTER TABLE allocations_new RENAME TO allocations;
+
+CREATE INDEX IF NOT EXISTS idx_allocations_pool_id ON allocations(pool_id);
+CREATE INDEX IF NOT EXISTS idx_allocations_status ON allocations(status);
+CREATE INDEX IF NOT EXISTS idx_allocations_pool_status ON allocations(pool_id, status);
+CREATE INDEX IF NOT EXISTS idx_allocations_created_at ON allocations(created_at DESC);
+
+CREATE UNIQUE INDEX IF NOT EXISTS idx_allocations_active_unique
+    ON allocations(pool_id, cidr)
+    WHERE status IN ('active', 'reserved');