feat(db): add playgroundPresets CRUD module

Implements listPlaygroundPresets, getPlaygroundPreset, createPlaygroundPreset,
updatePlaygroundPreset, and deletePlaygroundPreset using db.prepare() (never
raw db.exec or string interpolation). randomUUID() from node:crypto for IDs;
params serialized via JSON.stringify/JSON.parse with fallback to {}.

src/lib/db/playgroundPresets.ts

@@ -0,0 +1,167 @@
+/**
+ * db/playgroundPresets.ts — Playground Studio preset persistence.
+ *
+ * CRUD operations for the playground_presets table (migration 076).
+ * All queries use db.prepare() (better-sqlite3) — never raw db.exec() or
+ * string interpolation.
+ *
+ * @module lib/db/playgroundPresets
+ */
+
+import { getDbInstance } from "./core";
+import { randomUUID } from "node:crypto";
+
+// TODO(F1-merge): swap to import from "@/shared/schemas/playground" after F1 lands
+export interface PlaygroundPresetListItem {
+  id: string;
+  name: string;
+  endpoint: string;
+  model: string;
+  system: string | null;
+  params: Record<string, unknown>;
+  created_at: string;
+}
+
+type PlaygroundPresetRow = {
+  id: string;
+  name: string;
+  endpoint: string;
+  model: string;
+  system: string | null;
+  params_json: string;
+  created_at: string;
+};
+
+function rowToItem(row: PlaygroundPresetRow): PlaygroundPresetListItem {
+  let params: Record<string, unknown> = {};
+  try {
+    const parsed = JSON.parse(row.params_json);
+    if (parsed !== null && typeof parsed === "object" && !Array.isArray(parsed)) {
+      params = parsed as Record<string, unknown>;
+    }
+  } catch {
+    params = {};
+  }
+  return {
+    id: row.id,
+    name: row.name,
+    endpoint: row.endpoint,
+    model: row.model,
+    system: row.system,
+    params,
+    created_at: row.created_at,
+  };
+}
+
+/**
+ * Returns all presets ordered by created_at descending (newest first).
+ */
+export function listPlaygroundPresets(): PlaygroundPresetListItem[] {
+  const db = getDbInstance();
+  const rows = db
+    .prepare("SELECT * FROM playground_presets ORDER BY created_at DESC")
+    .all() as PlaygroundPresetRow[];
+  return rows.map(rowToItem);
+}
+
+/**
+ * Returns a single preset by id, or null when not found.
+ */
+export function getPlaygroundPreset(id: string): PlaygroundPresetListItem | null {
+  const db = getDbInstance();
+  const row = db
+    .prepare("SELECT * FROM playground_presets WHERE id = ? LIMIT 1")
+    .get(id) as PlaygroundPresetRow | undefined;
+  if (!row) return null;
+  return rowToItem(row);
+}
+
+/**
+ * Creates a new preset. Generates a UUID v4 for the id.
+ * Returns the persisted row via getPlaygroundPreset.
+ */
+export function createPlaygroundPreset(input: {
+  name: string;
+  endpoint: string;
+  model: string;
+  system: string | null | undefined;
+  params: Record<string, unknown>;
+}): PlaygroundPresetListItem {
+  const db = getDbInstance();
+  const id = randomUUID();
+  const params_json = JSON.stringify(input.params ?? {});
+  const system = input.system ?? null;
+
+  db.prepare(
+    "INSERT INTO playground_presets (id, name, endpoint, model, system, params_json) VALUES (?, ?, ?, ?, ?, ?)"
+  ).run(id, input.name, input.endpoint, input.model, system, params_json);
+
+  const created = getPlaygroundPreset(id);
+  // created cannot be null here — we just inserted the row
+  return created as PlaygroundPresetListItem;
+}
+
+/**
+ * Updates only the supplied fields on an existing preset.
+ * Returns the updated row, or null when the id does not exist.
+ */
+export function updatePlaygroundPreset(
+  id: string,
+  patch: Partial<{
+    name: string;
+    endpoint: string;
+    model: string;
+    system: string | null;
+    params: Record<string, unknown>;
+  }>
+): PlaygroundPresetListItem | null {
+  const db = getDbInstance();
+
+  // Verify row exists before building the dynamic UPDATE
+  const existing = getPlaygroundPreset(id);
+  if (!existing) return null;
+
+  const setClauses: string[] = [];
+  const values: unknown[] = [];
+
+  if (patch.name !== undefined) {
+    setClauses.push("name = ?");
+    values.push(patch.name);
+  }
+  if (patch.endpoint !== undefined) {
+    setClauses.push("endpoint = ?");
+    values.push(patch.endpoint);
+  }
+  if (patch.model !== undefined) {
+    setClauses.push("model = ?");
+    values.push(patch.model);
+  }
+  if ("system" in patch) {
+    setClauses.push("system = ?");
+    values.push(patch.system ?? null);
+  }
+  if (patch.params !== undefined) {
+    setClauses.push("params_json = ?");
+    values.push(JSON.stringify(patch.params));
+  }
+
+  if (setClauses.length === 0) {
+    // Empty patch — return current row unchanged
+    return existing;
+  }
+
+  values.push(id);
+  db.prepare(`UPDATE playground_presets SET ${setClauses.join(", ")} WHERE id = ?`).run(...values);
+
+  return getPlaygroundPreset(id);
+}
+
+/**
+ * Deletes a preset by id.
+ * Returns true when a row was deleted, false when the id did not exist.
+ */
+export function deletePlaygroundPreset(id: string): boolean {
+  const db = getDbInstance();
+  const result = db.prepare("DELETE FROM playground_presets WHERE id = ?").run(id);
+  return result.changes > 0;
+}