Phase 2: SQLite persistence, admin setup-link, login/sessions

Implements the secure single-admin panel entry (spec 7.6).

- internal/store: modernc.org/sqlite (pure Go, static build), WAL +
  foreign keys, embedded PRAGMA user_version migrations; schema 0001
  covers admin/settings/domains/applications/send_log/rate_limits (spec 9).
- Setup secret-link (spec 7.6.1): 128-bit crypto/rand token, printed to
  log + /data/setup-token (0600), 10-min TTL with regeneration, per-IP
  rate limit, subtle.ConstantTimeCompare, failures don't invalidate,
  one-time admin form, permanent invalidation once admin exists (/setup 404).
- bcrypt admin password; server-side username/password validation.
- Login + in-memory sessions, crypto-random token, cookie
  HttpOnly/Secure/SameSite (Secure toggleable for dev HTTP), login
  rate limit, auth middleware.
- html/template base layout + setup/login/dashboard, vendored htmx 2.0.4.
- build/entrypoint.sh: fix bind-mounted /data ownership as root before
  supervisord drops to the unprivileged panel user (found via container test).

Verified on selfpost.mixfed.ru: go vet/build/test/gofmt clean; e2e curl
of setup+login flows; docker build + run with -v ./data:/data creates the
DB and 0600 token owned by panel.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

internal/web/templates/setup.html

@@ -0,0 +1,20 @@
+{{define "content"}}
+<h1>Create administrator</h1>
+<div class="card narrow">
+  <p class="muted">This one-time link creates the single panel administrator.
+  After you submit, the link stops working for good.</p>
+  {{if .Error}}<p class="error">{{.Error}}</p>{{end}}
+  <form method="post" action="/setup/{{.Token}}">
+    <label for="username">Username</label>
+    <input id="username" name="username" autocomplete="username" autofocus required>
+
+    <label for="password">Password</label>
+    <input id="password" name="password" type="password" autocomplete="new-password" required>
+
+    <label for="password_confirm">Confirm password</label>
+    <input id="password_confirm" name="password_confirm" type="password" autocomplete="new-password" required>
+
+    <button type="submit">Create administrator</button>
+  </form>
+</div>
+{{end}}