fix: various security fixes (#38406)

Addresses a batch of privately reported security issues, grouped by
area:

- **SSRF** - migration PR-patch/asset fetches, OAuth2 avatar & OpenID
discovery, pull-mirror URL re-validation, and the outbound proxy path.
- **Access-token scope** - prevent scope escalation on token creation;
keep public-only tokens confined (feeds, packages, Actions listings,
star/watch lists, limited/private owners).
- **Access control / disclosure** - go-get default-branch leak, webhook
authorization-header leak, watch clearing on private transitions,
label/attachment scoping.
- **Denial of service** - input bounds for npm dist-tags, Debian control
files, Arch file lists, and SSH keys.

### 📌 Attention for site admins

Not breaking - existing configs keep working - but two changes are worth
a look:

- **New SSRF protection** Outbound requests (migrations, OAuth2 avatars,
OpenID discovery, pull mirrors, proxy path) are now validated against
the allow/block host lists. If your instance legitimately reaches
internal hosts, you may need to add them to
`[security].ALLOWED_HOST_LIST` (and the relevant `ALLOW_LOCALNETWORKS`
settings).
- **Deprecation** `[webhook].ALLOWED_HOST_LIST` is deprecated and will
be removed in a future release. Use `[security].ALLOWED_HOST_LIST`
instead; the old key still works for now.

---------

Co-authored-by: TheFox0x7 <thefox0x7@gmail.com>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Zettat123 <zettat123@gmail.com>

services/repository/repository.go

@@ -273,6 +273,11 @@ func updateRepository(ctx context.Context, repo *repo_model.Repository, visibili
 			if err = repo_model.ClearRepoStars(ctx, repo.ID); err != nil {
 				return err
 			}
+
+			// watchers who lost access must not keep watching the now-private repo
+			if err = repo_model.ClearRepoWatches(ctx, repo.ID); err != nil {
+				return err
+			}
 		}
 
 		// Create/Remove git-daemon-export-ok for git-daemon...

services/repository/repository_test.go

@@ -90,3 +90,26 @@ func TestMakeRepoPrivateClearsWatches(t *testing.T) {
 	assert.True(t, updatedRepo.IsPrivate)
 	assert.Zero(t, updatedRepo.NumWatches)
 }
+
+// TestUpdateRepositoryClearsWatchesOnVisibilityChange ensures the shared updateRepository
+// helper (used by the API EditRepo path) also clears watches when a repo goes private.
+func TestUpdateRepositoryClearsWatchesOnVisibilityChange(t *testing.T) {
+	assert.NoError(t, unittest.PrepareTestDatabase())
+
+	repo := unittest.AssertExistsAndLoadBean(t, &repo_model.Repository{ID: 1})
+	assert.False(t, repo.IsPrivate)
+
+	watchers, err := repo_model.GetRepoWatchersIDs(t.Context(), repo.ID)
+	require.NoError(t, err)
+	require.NotEmpty(t, watchers)
+
+	repo.IsPrivate = true
+	require.NoError(t, updateRepository(t.Context(), repo, true))
+
+	watchers, err = repo_model.GetRepoWatchersIDs(t.Context(), repo.ID)
+	assert.NoError(t, err)
+	assert.Empty(t, watchers)
+
+	updatedRepo := unittest.AssertExistsAndLoadBean(t, &repo_model.Repository{ID: repo.ID})
+	assert.Zero(t, updatedRepo.NumWatches)
+}