Refactor sign-in page and IAM service lifecycle

Redesign the sign-in page to show email/password form inline
with OIDC provider buttons (with vendor icons) instead of
separate pages. Extract OIDCProvider type to its own file.

Replace errgroup with sync.WaitGroup + WithCancelCause for
graceful shutdown in IAM services. Refactor garbage collectors
to use functional options and time.Ticker instead of
time.After to avoid repeated allocations.

Signed-off-by: Bryan Frimin <bryan@getprobo.com>

pkg/coredata/oidc_provider.go

@@ -0,0 +1,46 @@
+// Copyright (c) 2025 Probo Inc <hello@getprobo.com>.
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+// REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+// AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+// INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+// LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
+// OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+// PERFORMANCE OF THIS SOFTWARE.
+
+package coredata
+
+import "fmt"
+
+type OIDCProvider string
+
+const (
+	OIDCProviderGoogle    OIDCProvider = "GOOGLE"
+	OIDCProviderMicrosoft OIDCProvider = "MICROSOFT"
+)
+
+func (p OIDCProvider) IsValid() bool {
+	switch p {
+	case OIDCProviderGoogle, OIDCProviderMicrosoft:
+		return true
+	}
+	return false
+}
+
+func (p OIDCProvider) String() string { return string(p) }
+
+func (p *OIDCProvider) UnmarshalText(text []byte) error {
+	*p = OIDCProvider(text)
+	if !p.IsValid() {
+		return fmt.Errorf("%s is not a valid OIDCProvider", string(text))
+	}
+	return nil
+}
+
+func (p OIDCProvider) MarshalText() ([]byte, error) {
+	return []byte(p.String()), nil
+}