GRM-20: refactor(scripts): centralize constants, API clients, and HTTP status codes

- Add shared config.py with API URLs, regexes, timeouts, pagination
- Add GiteaClient and VikunjaClient in api_clients.py with pooled sessions
- Add APIError exception for unified HTTP error handling
- Refactor all scripts to use shared modules and http.HTTPStatus
- Rewrite unit tests to mock clients and use HTTPStatus constants
- Add tests for api_clients and config modules
- Achieve 100% test coverage

tests/unit/test_config.py

@@ -0,0 +1,54 @@
+"""Unit tests for config module constants."""
+
+from gitea_runner_manager.config import (
+    BRANCH_PROTECTION_CONFIG,
+    CONVENTIONAL_RE,
+    DEFAULT_PER_PAGE,
+    DEFAULT_TIMEOUT,
+    GITEA_API_URL,
+    LABEL_CONFIG,
+    REPO_NAME,
+    REPO_OWNER,
+    TASK_ID_RE,
+    VIKUNJA_API_URL,
+    VIKUNJA_PROJECT_ID,
+)
+
+
+class TestConfigConstants:
+    def test_api_urls(self) -> None:
+        assert GITEA_API_URL == "https://git.oblachno.oblachno.fyi/api/v1"
+        assert VIKUNJA_API_URL == "https://work.oblachno.oblachno.fyi/api/v1"
+
+    def test_project_ids(self) -> None:
+        assert VIKUNJA_PROJECT_ID == 6
+
+    def test_timeouts(self) -> None:
+        assert DEFAULT_TIMEOUT == 30
+        assert DEFAULT_PER_PAGE == 50
+
+    def test_owner_and_repo(self) -> None:
+        assert REPO_OWNER == "oblachno-oss"
+        assert REPO_NAME == "grm"
+
+    def test_task_id_re(self) -> None:
+        assert TASK_ID_RE.search("GRM-1")
+        assert TASK_ID_RE.search("GRM-123")
+        assert not TASK_ID_RE.search("GRM-")
+        assert not TASK_ID_RE.search("other text")
+
+    def test_conventional_re(self) -> None:
+        assert CONVENTIONAL_RE.match("feat: add feature")
+        assert CONVENTIONAL_RE.match("fix(scope): bug fix")
+        assert not CONVENTIONAL_RE.match("random message")
+        assert not CONVENTIONAL_RE.match("feat:")
+
+    def test_branch_protection_config(self) -> None:
+        assert BRANCH_PROTECTION_CONFIG["branch_name"] == "master"
+        assert BRANCH_PROTECTION_CONFIG["enable_push"] is False
+        assert BRANCH_PROTECTION_CONFIG["required_approvals"] == 1
+        assert BRANCH_PROTECTION_CONFIG["status_check_contexts"] == ["lint", "unit-tests", "molecule-tests"]
+
+    def test_label_config(self) -> None:
+        assert LABEL_CONFIG["name"] == "ready-to-merge"
+        assert LABEL_CONFIG["color"] == "2ecc71"