Add notification channels and a provider abstraction

Users get a notification_channels table (replacing users.ntfy_topic); pages fan
out to every enabled channel through a notify.Provider registry, with ntfy as
the first provider. Fallbacks are typed {type, address}. Schema edited in place:
existing databases must be recreated.

Claude-Session: https://claude.ai/code/session_016mBLURvJoMuUEr9cB2RpUN

internal/api/admin_test.go

@@ -7,6 +7,8 @@ import (
 	"net/http"
 	"strconv"
 	"testing"
+
+	"git.ryuvia.com/niklas/terdut-server/internal/api"
 )
 
 // The bootstrap user is an administrator; everybody it creates afterwards is
@@ -96,8 +98,9 @@ func TestAdmin_MemberIsRefusedAdministration(t *testing.T) {
 			map[string]string{"password": "hunter2-hunter2"}},
 		{"mint a key for the admin", http.MethodPost, "/api/users/1/api-keys",
 			map[string]string{"name": "borrowed"}},
-		{"retarget the admin's notifications", http.MethodPut, "/api/users/1/notify",
-			map[string]string{"ntfy_topic": "attacker-topic"}},
+		{"add a channel to the admin's notifications", http.MethodPost, "/api/users/1/channels",
+			map[string]string{"type": "ntfy", "address": "attacker-topic"}},
+		{"read the admin's channels", http.MethodGet, "/api/users/1/channels", nil},
 	}
 
 	for _, c := range cases {
@@ -111,14 +114,14 @@ func TestAdmin_MemberIsRefusedAdministration(t *testing.T) {
 
 // Being refused other people's accounts must not cost a user their own.
 func TestAdmin_MemberKeepsTheirOwnAccount(t *testing.T) {
-	s := newTS(t)
+	s := newTS(t, api.NotifyConfig{Providers: newFakeNtfy(t).providers()})
 	memberID, call := member(t, s, "member")
 	self := "/api/users/" + id64(memberID)
 
-	resp := call(http.MethodPut, self+"/notify", map[string]string{"ntfy_topic": "terdut-member"})
+	resp := call(http.MethodPost, self+"/channels", map[string]string{"type": "ntfy", "address": "terdut-member"})
 	resp.Body.Close()
-	if resp.StatusCode != http.StatusOK {
-		t.Errorf("own notify target: %d", resp.StatusCode)
+	if resp.StatusCode != http.StatusCreated {
+		t.Errorf("own notification channel: %d", resp.StatusCode)
 	}
 
 	resp = call(http.MethodPut, self+"/password", map[string]string{"password": "correct-horse-battery"})