feat: add username/hostname support with IRC hostmask format

- Add username and hostname columns to sessions table (001_initial.sql)
- Accept optional username field in session creation and registration
  endpoints; defaults to nick if not provided
- Resolve hostname via reverse DNS of connecting client IP at session
  creation time (supports X-Forwarded-For and X-Real-IP headers)
- Display real username and hostname in WHOIS (311 RPL_WHOISUSER) and
  WHO (352 RPL_WHOREPLY) responses instead of nick/servername
- Add FormatHostmask helper for nick!user@host format
- Add SessionHostInfo type and GetSessionHostInfo query
- Include username/hostname in MemberInfo and ChannelMembers results
- Extract validateHashcash and resolveUsername helpers to stay under
  funlen limits
- Add comprehensive unit tests for all new DB functions, hostmask
  formatting, and integration tests for WHOIS/WHO responses
- Update README with hostmask documentation, new API fields, and
  updated schema reference

internal/db/auth_test.go

@@ -13,7 +13,7 @@ func TestRegisterUser(t *testing.T) {
 	ctx := t.Context()
 
 	sessionID, clientID, token, err :=
-		database.RegisterUser(ctx, "reguser", "password123")
+		database.RegisterUser(ctx, "reguser", "password123", "", "")
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -38,6 +38,69 @@ func TestRegisterUser(t *testing.T) {
 	}
 }
 
+func TestRegisterUserWithUserHost(t *testing.T) {
+	t.Parallel()
+
+	database := setupTestDB(t)
+	ctx := t.Context()
+
+	sessionID, _, _, err := database.RegisterUser(
+		ctx, "reguhost", "password123",
+		"myident", "example.org",
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	info, err := database.GetSessionHostInfo(
+		ctx, sessionID,
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if info.Username != "myident" {
+		t.Fatalf(
+			"expected myident, got %s", info.Username,
+		)
+	}
+
+	if info.Hostname != "example.org" {
+		t.Fatalf(
+			"expected example.org, got %s",
+			info.Hostname,
+		)
+	}
+}
+
+func TestRegisterUserDefaultUsername(t *testing.T) {
+	t.Parallel()
+
+	database := setupTestDB(t)
+	ctx := t.Context()
+
+	sessionID, _, _, err := database.RegisterUser(
+		ctx, "regdefault", "password123", "", "",
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	info, err := database.GetSessionHostInfo(
+		ctx, sessionID,
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if info.Username != "regdefault" {
+		t.Fatalf(
+			"expected regdefault, got %s",
+			info.Username,
+		)
+	}
+}
+
 func TestRegisterUserDuplicateNick(t *testing.T) {
 	t.Parallel()
 
@@ -45,7 +108,7 @@ func TestRegisterUserDuplicateNick(t *testing.T) {
 	ctx := t.Context()
 
 	regSID, regCID, regToken, err :=
-		database.RegisterUser(ctx, "dupnick", "password123")
+		database.RegisterUser(ctx, "dupnick", "password123", "", "")
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -55,7 +118,7 @@ func TestRegisterUserDuplicateNick(t *testing.T) {
 	_ = regToken
 
 	dupSID, dupCID, dupToken, dupErr :=
-		database.RegisterUser(ctx, "dupnick", "other12345")
+		database.RegisterUser(ctx, "dupnick", "other12345", "", "")
 	if dupErr == nil {
 		t.Fatal("expected error for duplicate nick")
 	}
@@ -72,7 +135,7 @@ func TestLoginUser(t *testing.T) {
 	ctx := t.Context()
 
 	regSID, regCID, regToken, err :=
-		database.RegisterUser(ctx, "loginuser", "mypassword")
+		database.RegisterUser(ctx, "loginuser", "mypassword", "", "")
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -110,7 +173,7 @@ func TestLoginUserWrongPassword(t *testing.T) {
 	ctx := t.Context()
 
 	regSID, regCID, regToken, err :=
-		database.RegisterUser(ctx, "wrongpw", "correctpass")
+		database.RegisterUser(ctx, "wrongpw", "correctpass", "", "")
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -138,7 +201,7 @@ func TestLoginUserNoPassword(t *testing.T) {
 
 	// Create anonymous session (no password).
 	anonSID, anonCID, anonToken, err :=
-		database.CreateSession(ctx, "anon")
+		database.CreateSession(ctx, "anon", "", "")
 	if err != nil {
 		t.Fatal(err)
 	}