feat: redirect root path based on auth state (#52)

Closes #51

The root path `/` now checks for an authenticated session and redirects accordingly:

- **Authenticated users** → `303 See Other` redirect to `/sources` (the webhook dashboard)
- **Unauthenticated users** → `303 See Other` redirect to `/pages/login`

### Changes

- **`internal/handlers/index.go`** — Replaced the template-rendering `HandleIndex()` with a session-checking redirect handler. Removed `formatUptime` helper (dead code after this change).
- **`internal/handlers/handlers.go`** — Removed `index.html` from the template map (no longer rendered).
- **`internal/handlers/handlers_test.go`** — Replaced the old "handler is not nil" test with two proper redirect tests:
  - `unauthenticated redirects to login` — verifies 303 to `/pages/login`
  - `authenticated redirects to sources` — sets up an authenticated session cookie, verifies 303 to `/sources`
  - Removed `TestFormatUptime` (tested dead code).
- **`README.md`** — Updated the API endpoints table to describe the new redirect behavior.

### How it works

The handler calls `session.Get(r)` and `session.IsAuthenticated(sess)` — the same pattern used by the `RequireAuth` middleware and `HandleLoginPage`. No new dependencies or session logic introduced.

The login flow is unaffected: `HandleLoginSubmit` redirects to `/` after successful login, which now forwards to `/sources` (one extra redirect hop, but correct and clean).

Co-authored-by: clawbot <clawbot@noreply.git.eeqj.de>
Co-authored-by: clawbot <clawbot@eeqj.de>
Reviewed-on: #52
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

internal/handlers/handlers.go

@@ -65,7 +65,6 @@ func New(lc fx.Lifecycle, params HandlersParams) (*Handlers, error) {
 
 	// Parse all page templates once at startup
 	s.templates = map[string]*template.Template{
-		"index.html":         parsePageTemplate("index.html"),
 		"login.html":         parsePageTemplate("login.html"),
 		"profile.html":       parsePageTemplate("profile.html"),
 		"sources_list.html":  parsePageTemplate("sources_list.html"),