feat(db): add playground_presets migration 076

Creates playground_presets table with indexes on name and endpoint.
Idempotent via IF NOT EXISTS (migration 076 per group-C D2 decision).

src/lib/db/migrations/076_playground_presets.sql

@@ -0,0 +1,15 @@
+CREATE TABLE IF NOT EXISTS playground_presets (
+  id TEXT PRIMARY KEY,
+  name TEXT NOT NULL,
+  endpoint TEXT NOT NULL,
+  model TEXT NOT NULL,
+  system TEXT,
+  params_json TEXT NOT NULL DEFAULT '{}',
+  created_at TEXT NOT NULL DEFAULT CURRENT_TIMESTAMP
+);
+
+CREATE INDEX IF NOT EXISTS idx_playground_presets_name
+  ON playground_presets(name);
+
+CREATE INDEX IF NOT EXISTS idx_playground_presets_endpoint
+  ON playground_presets(endpoint);