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.

bot_bottle/dlp_detectors.py

@@ -3,9 +3,8 @@
 Pure Python, no mitmproxy dependency. Each detector is a module-level
 function returning `ScanResult | None`.
 
-Ships flat into the gateway image alongside
-`egress_addon_core.py` — both this file and the package source use
-the same try/except import shim pattern.
+Available in the gateway via the installed `bot_bottle` package
+(see `Dockerfile.gateway`).
 """
 
 from __future__ import annotations
@@ -20,10 +19,7 @@ from math import log2
 from collections import Counter
 from urllib.parse import quote as url_quote
 
-try:
-    from egress_addon_core import ScanResult  # type: ignore[import-not-found]
-except ImportError:  # pragma: no cover - host-side path
-    from .egress_addon_core import ScanResult
+from .egress_addon_core import ScanResult
 
 
 # ---------------------------------------------------------------------------