DC-042: implement real Tailscale manager — replace null stub

The previous getTailscaleStatus() in src/app.js was a hard-coded
`return null` stub with a TODO saying it would be populated by context.
The context had a tailscale.* namespace declared with null function
stubs (routes/context.js:71), but nothing ever set them to real
functions. routes/tailscale.js has been calling ctx.tailscale.getStatus()
/ getLocalIP() / isTailscaleIP() and getting undefined back, silently
returning empty device lists. The tailscaleAuthMiddleware's allowedTailnet
check (DC-121, device-not-in-tailnet 403) was dead code for the same reason.

This commit replaces the stub with a real implementation:

- New src/managers/tailscale-manager.js shells out to the host's
  `tailscale status --json` (cached 5 minutes), parses the result, and
  exposes getStatus / getLocalIP / getSummary / getDevices / isTailscaleIP /
  invalidateCache / getAccessToken (stub) / startSyncTimer / stopSyncTimer
  / syncAPI (stub). All failure modes (CLI missing, tailscaled down,
  malformed JSON, EACCES) are handled gracefully — return null with no
  cache poisoning.
- src/context/index.js now wires the manager into ctx.tailscale.* so
  routes/tailscale.js and middleware.js's allowedTailnet gate get the
  real functions.
- src/app.js:189 getTailscaleStatus() now delegates to the manager
  instead of returning null.
- The duplicate isTailscaleIP() in src/app.js:179 (no malformed-input
  guards) is removed in favor of the canonical version in
  src/utilities/network-detector.js (DC-031) which the manager also uses.
- start.sh now bind-mounts /usr/bin/tailscale (statically linked Go binary
  — works under Alpine libc) and /var/run/tailscale/ into the container,
  read-only. Lets the container invoke the CLI without needing its own
  tailscale install.
- 41 new unit tests in __tests__/tailscale-manager.test.js cover: CLI
  success/missing/daemon-down/malformed-JSON paths, 5-min cache hit/miss,
  1-hour installed-cache hit/miss, getLocalIP IPv4/IPv6/missing-choices,
  getSummary shape, getDevices shape with full + minimal peer fields,
  startSyncTimer/stopSyncTimer interval + idempotency, TAILSCALE_BIN env
  override.

Total: 1138 tests pass (was 1097, +41 new), 0 new ESLint warnings.

What this unlocks:
- /api/v1/tailscale/status → real installed/connected/hostname/ip/
  peerCount/onlinePeerCount summary instead of empty
- /api/v1/tailscale/devices → real device list (was returning [])
- /api/v1/tailscale/check-connection → works (uses real isTailscaleIP)
- tailscaleAuthMiddleware allowedTailnet check (DC-121) is no longer
  dead code — a request from a Tailscale IP not in the allowed tailnet
  now actually gets 403 instead of being silently allowed.

dashcaddy-api/src/app.js

@@ -176,20 +176,16 @@ async function createApp() {
     return typeof id === 'string' && CONTAINER_ID_RE.test(id);
   }
 
-  function isTailscaleIP(ip) {
-    if (!ip) return false;
-    const parts = ip.split('.');
-    if (parts.length !== 4) return false;
-    const first = parseInt(parts[0]);
-    const second = parseInt(parts[1]);
-    return first === 100 && second >= 64 && second <= 127;
-  }
+  // Tailscale CGNAT classification. Imported from network-detector.js (DC-031)
+  // so there's one source of truth — the local copy here had no malformed-input
+  // guards and would return false on NaN silently.
+  const { isTailscaleIP } = require('./utilities/network-detector');
 
-  // eslint-disable-next-line require-await -- stub for now, will gain await when wired into context
-  async function getTailscaleStatus() {
-    // Stub for now - will be populated by context
-    return null;
-  }
+  // Real implementation — delegates to the tailscale manager which
+  // shells out to the host's `tailscale status --json` (cached 5min).
+  // Kept here as a top-level function for back-compat with middleware.js
+  // and any other call site that imports it via the createApp() factory.
+  const { getStatus: getTailscaleStatus } = require('./managers/tailscale-manager');
 
   // Back-compat: reverse-proxy SSO snippets (Caddy forward_auth + per-service
   // auto-login pages) historically call these endpoints under the pre-1.5.0