Replace Python endpoint-discovery with --discover flag (replaces #936)

- Add --discover and --discover-output to unpoller; uses first unifi
  controller from config to probe known API endpoints and write a
  shareable markdown report.
- Add Discoverer interface and RunDiscover(); inputunifi implements
  Discoverer via unifi.DiscoverEndpoints.
- Remove tools/endpoint-discovery/ (Python/Playwright).
- Add docs/PR_936_REPLACEMENT.md. .gitignore: test config and report.

Requires unpoller/unifi with DiscoverEndpoints (replace in go.mod until
unifi release).

docs/PR_936_REPLACEMENT.md

@@ -0,0 +1,70 @@
+# Replacement for PR #936: Endpoint discovery via `--discover` (Go)
+
+**Supersedes:** [PR #936](https://github.com/unpoller/unpoller/pull/936) (Python endpoint-discovery tool in `tools/endpoint-discovery/`).
+
+PR #936 added a Python/Playwright tool to discover API endpoints by driving a headless browser. This replacement implements the same goal **inside unpoller** using the unifi library: no Python, no Playwright, same config as normal polling.
+
+---
+
+## Summary
+
+- **Feature:** `--discover` flag on unpoller that probes known API endpoints on the controller and writes a shareable markdown report.
+- **Credentials:** Uses the same config file (and first unifi controller) as normal unpoller runs.
+- **Output:** Markdown file (default `api_endpoints_discovery.md`) with method, path, and HTTP status for each endpoint. Users can share this when reporting API/404 issues (e.g. [issue #935](https://github.com/unpoller/unpoller/issues/935)).
+- **Dependency:** Requires [unpoller/unifi](https://github.com/unpoller/unifi) with `DiscoverEndpoints` and `Probe` (unifi PR/branch with discovery support).
+
+---
+
+## Changes in unpoller (this PR)
+
+| File | Change |
+|------|--------|
+| `pkg/poller/config.go` | Add `Discover bool`, `DiscoverOutput string` to `Flags`. |
+| `pkg/poller/start.go` | Register `--discover` and `--discover-output`; when `--discover`, call `RunDiscover()` and exit. |
+| `pkg/poller/commands.go` | Add `RunDiscover()`: load config, init inputs, find input implementing `Discoverer`, call `Discover(outputPath)`. |
+| `pkg/poller/inputs.go` | Add optional interface `Discoverer` with `Discover(outputPath string) error`. |
+| `pkg/inputunifi/discover.go` | **New file.** Implement `Discoverer`: first controller, authenticate, get sites, call `c.Unifi.DiscoverEndpoints(site, outputPath)`. |
+| `.gitignore` | Add `up.discover-test.json`, `api_endpoints_discovery.md`. |
+
+**Removed (vs PR #936):** No `tools/endpoint-discovery/` (no Python, no Playwright).
+
+---
+
+## Usage
+
+```bash
+# Same config as normal unpoller (first unifi controller is used)
+unpoller --discover --config /path/to/up.conf --discover-output api_endpoints_discovery.md
+```
+
+If config is in the default search path:
+
+```bash
+unpoller --discover --discover-output api_endpoints_discovery.md
+```
+
+---
+
+## PR title (for replacement PR)
+
+**Add `--discover` to probe API endpoints and write shareable report (replaces #936)**
+
+---
+
+## PR description (suggested)
+
+**Replaces #936** (Python endpoint-discovery tool). Implements endpoint discovery inside unpoller using the unifi library; no Python or Playwright.
+
+**What it does**
+- `unpoller --discover` uses the first unifi controller from your config, authenticates, and probes a set of known API paths.
+- Writes a markdown report (default `api_endpoints_discovery.md`) with method, path, and HTTP status for each endpoint.
+- Same credentials as normal polling; users can share the file when reporting API/404 issues (e.g. #935).
+
+**Usage**
+```bash
+unpoller --discover --config /path/to/up.conf --discover-output api_endpoints_discovery.md
+```
+
+**Requires** unpoller/unifi with `DiscoverEndpoints` (and `Probe`) merged or a compatible unifi version.
+
+**Closes #936** (replaced by this approach).