implement backend components for push notifications

go/migrations/000016_push_tokens.down.sql

@@ -0,0 +1 @@
+DROP TABLE IF EXISTS push_tokens;

go/migrations/000016_push_tokens.up.sql

@@ -0,0 +1,10 @@
+CREATE TABLE push_tokens (
+    token        TEXT PRIMARY KEY,
+    human_id     TEXT NOT NULL REFERENCES humans(id) ON DELETE CASCADE,
+    platform     TEXT NOT NULL CHECK (platform IN ('ios', 'android')),
+    app_version  TEXT,
+    created_at   TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    last_seen_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE INDEX push_tokens_human_id_idx ON push_tokens (human_id);