refactor(gateway): replace flat-file import shims with installed package

Install bot_bottle via pip in Dockerfile.gateway instead of COPYing
individual .py files flat under /app/. This eliminates the try/except
import shims in egress_addon_core, dlp_detectors, egress_addon,
supervise, supervise_server, and git_http_backend that existed only
to support the flat-bundle layout.

Adds bot_bottle/constants.py as a single source of truth for
IDENTITY_HEADER and GIT_GATE_TIMEOUT_SECS, removing the duplicated
literal definitions in egress_addon.py, supervise_server.py,
git_http_backend.py, and git_gate_render.py.

Test files updated to match: test_supervise_server.py drops the
sys.path.insert hack in favour of direct package imports; the
egress_addon test shims no longer pre-populate sys.modules with a
bare egress_addon_core alias.

tests/unit/test_supervise_server.py

@@ -2,7 +2,6 @@
 
 import http.client
 import json
-import sys
 import tempfile
 import threading
 import time
@@ -13,15 +12,9 @@ from unittest.mock import patch
 
 from tests.unit import use_bottle_root
 
-
-# The server module loads `supervise` via same-directory import inside
-# the container (Dockerfile.supervise WORKDIRs into /app). For tests
-# we mirror that by injecting bot_bottle/ onto sys.path under the
-# bare name `supervise`.
-sys.path.insert(0, str(Path(__file__).resolve().parent.parent.parent / "bot_bottle"))
-import supervise as _sv  # noqa: E402  # type: ignore
-import queue_store as _qs  # noqa: E402  # type: ignore
-import audit_store as _as  # noqa: E402  # type: ignore
+from bot_bottle import supervise as _sv
+from bot_bottle import queue_store as _qs
+from bot_bottle import audit_store as _as
 
 from bot_bottle import supervise_server  # noqa: E402
 from bot_bottle.supervise_server import (