feat(report): Stage-0 usage gate counts from a baseline date (default 2026-06-11)

The return-usage gate (ADR-016) counted distinct active weeks over ALL history,
so pre-launch noise — testing churn and the period the pilot sat blocked on zero
summaries — would inflate the signal. Add a baseline: ActiveWeeks(ctx, since)
filters login_events + summary_actions to seen_at/acted_at >= since. The report
command sets it to TAPIR_USAGE_GATE_START (YYYY-MM-DD, default 2026-06-11 — the
morning the pilot was unblocked) and prints the baseline. The gate now measures
whether users RETURN once it genuinely works.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/homelab-integration.md

@@ -224,6 +224,9 @@ knobs plus one load-bearing deployment constraint:
 - `TAPIR_DISCOVERY_INTERVAL` — Go duration, e.g. `2h`. The cadence the serve process runs a
   discovery pass for every registered user (run-once-on-startup, then every interval).
   **Unset or `0` = disabled** (dev/tests never auto-fetch).
+- `TAPIR_USAGE_GATE_START` — `YYYY-MM-DD`, default **`2026-06-11`** (the morning the pilot was
+  unblocked and summaries started flowing). `tapir report` counts return-usage (distinct active
+  weeks, ADR-016) only from this date, so pre-launch testing and the blocked period are excluded.
 - `TAPIR_FETCH_RATE` — Go duration, default `2s`. The **process-wide per-egress-IP caption-fetch
   rate gate** (ADR-014 item 2). Every caption fetch — scheduler runners *and* the web "Summarize"
   click-path — serialises through this one limiter so the pod cannot collectively trip 429s. `0`