feat(intake-links): per-(link, project) daily-cap pure helpers

Phase 9a Task 2. Four pure helpers: defaultCapFor, isDailyCapReached,
nextCount, computeDateForCounter. 7 unit tests cover the caps
boundary (cap=0 means 'no submissions today'; todayCount === cap
is reached), the count increment, and the local-date format.

Note: project convention puts unit tests next to source
(src/**/*.test.ts), not under tests/unit/. Adjusted from the plan.

src/lib/intake-links/submissions.test.ts

@@ -0,0 +1,48 @@
+import { describe, it, expect } from "vitest";
+
+import {
+  computeDateForCounter,
+  defaultCapFor,
+  isDailyCapReached,
+  nextCount,
+} from "@/lib/intake-links/submissions";
+
+describe("defaultCapFor", () => {
+  it("returns the row cap when set, otherwise 100", () => {
+    expect(defaultCapFor(0)).toBe(100);
+    expect(defaultCapFor(50)).toBe(50);
+    expect(defaultCapFor(10000)).toBe(10000);
+  });
+
+  it("treats undefined as 'use default'", () => {
+    expect(defaultCapFor(undefined)).toBe(100);
+  });
+});
+
+describe("isDailyCapReached", () => {
+  it("is false when the counter is below the cap", () => {
+    expect(isDailyCapReached(0, 100)).toBe(false);
+    expect(isDailyCapReached(99, 100)).toBe(false);
+  });
+  it("is true when the counter equals or exceeds the cap", () => {
+    expect(isDailyCapReached(100, 100)).toBe(true);
+    expect(isDailyCapReached(101, 100)).toBe(true);
+  });
+  it("treats cap=0 as 'no submissions today'", () => {
+    expect(isDailyCapReached(0, 0)).toBe(true);
+  });
+});
+
+describe("nextCount", () => {
+  it("increments by one", () => {
+    expect(nextCount(0)).toBe(1);
+    expect(nextCount(7)).toBe(8);
+  });
+});
+
+describe("computeDateForCounter", () => {
+  it("formats a Date as YYYY-MM-DD in local time", () => {
+    const d = new Date(2026, 8, 21, 12, 0, 0);
+    expect(computeDateForCounter(d)).toBe("2026-09-21");
+  });
+});

src/lib/intake-links/submissions.ts

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+/**
+ * Per-(link, project) daily-cap helpers for the Phase 9 picker path.
+ *
+ * Phase 8 stored `submission_count_today` on the link row itself, which
+ * silently meant "across all projects on this link". With one link per
+ * client, that cap stops being meaningful — one busy project could lock
+ * the rest of the client's projects out. Phase 9 moves the counter onto a
+ * new `intake_link_submissions` collection, one row per
+ * (link, project, date), so each project has its own daily budget.
+ *
+ * These helpers are pure. The PocketBase reads/writes live in
+ * `src/lib/intake-links/index.ts` next to `resolveIntakeLink`.
+ */
+
+export const DEFAULT_DAILY_CAP = 100;
+
+/** Resolve the effective cap from the row's optional field. */
+export function defaultCapFor(rowCap: number | undefined): number {
+  if (typeof rowCap === "number" && rowCap > 0) return rowCap;
+  return DEFAULT_DAILY_CAP;
+}
+
+/** True when today's count has reached or exceeded the cap. */
+export function isDailyCapReached(todayCount: number, cap: number): boolean {
+  if (cap <= 0) return true; // explicit zero means "no submissions today"
+  return todayCount >= cap;
+}
+
+/** Increment. Pure. */
+export function nextCount(todayCount: number): number {
+  return todayCount + 1;
+}
+
+/** Local-date string in YYYY-MM-DD form. Same shape as the Phase 8 helper. */
+export function computeDateForCounter(now: Date = new Date()): string {
+  const y = now.getFullYear();
+  const m = String(now.getMonth() + 1).padStart(2, "0");
+  const d = String(now.getDate()).padStart(2, "0");
+  return `${y}-${m}-${d}`;
+}