Route container proxies through the leg that is actually reachable

The workspace editor hung for 60s and then 504'd. Three independent faults were
stacked behind that one symptom.

Reachability: editor_target delegated to proxy_target, which returns
CONTAINER_PROXY_HOST plus the published host port and never falls back to the
container. From inside the app container that address crosses docker0 into the
host INPUT chain, whose policy is DROP with an allow-list that does not include
the published port range, so the packet was dropped and the request hung rather
than being refused. Measured from the app container: container_ip:8443 answers
302, gateway:20006 is dropped. One shared reachable_target now prefers the
direct container leg and falls back to the published port, and editor_target
uses tunnel_target as services/containers/CLAUDE.md already required. The same
defect affected /p/{slug} ingress and every tunnel, since all three resolved
through proxy_target.

The recorded measurement that motivated the old order (container_ip times out,
gateway connects) no longer holds: make docker-attach puts the app on the
instances' bridge network, which is what makes the direct leg work.

Duplicate response headers: the forwarding core relayed the upstream Date and
Server alongside the ones the serving layer generates, so every proxied
response carried two of each. Both are singleton headers and duplicating them
is malformed HTTP.

Serialization: WorkspaceViewOut declared flag_reason and three sibling strings
as str, so a NULL column made the workspace page 500 for JSON clients.

Documents the two public hostnames and the devplace.net SSH tunnel, so a future
session does not conclude the site is down after pointing curl --resolve at an
address the hostname does not resolve to, and adds the layered procedure for
diagnosing a production failure.

Verified on production with Playwright over both hostnames: the code-server
login renders and the workbench loads. Suite: 3345 passed.

tests/unit/services/containers/api.py

@@ -507,3 +507,45 @@ def test_raw_query_survives_a_hash_in_the_path():
     request.scope["path"] = "/weird/a b#c"
     assert forward.raw_query(request) == "keep=1"
     assert request.url.query == ""
+
+
+def test_tunnel_target_prefers_the_direct_container_leg():
+    instance = {
+        "ports_json": '[{"host": 20500, "container": 8443, "proto": "tcp"}]',
+        "container_gateway": "172.17.0.1",
+        "container_ip": "172.17.0.9",
+    }
+    assert api.tunnel_target(instance, 8443) == ("172.17.0.9", 8443)
+
+
+def test_tunnel_target_falls_back_to_the_published_port_without_a_container_ip():
+    instance = {
+        "ports_json": '[{"host": 20500, "container": 8443, "proto": "tcp"}]',
+        "container_gateway": "172.17.0.1",
+    }
+    assert api.tunnel_target(instance, 8443) == ("172.17.0.1", 20500)
+
+
+def test_proxy_target_and_tunnel_target_agree_on_the_same_port():
+    instance = {
+        "ports_json": '[{"host": 20500, "container": 8443, "proto": "tcp"}]',
+        "container_gateway": "172.17.0.1",
+        "container_ip": "172.17.0.9",
+        "ingress_port": 8443,
+    }
+    assert api.proxy_target(instance) == api.tunnel_target(instance, 8443)
+
+
+def test_tunnel_target_dials_the_container_for_an_unpublished_port():
+    instance = {
+        "ports_json": '[{"host": 20500, "container": 8443, "proto": "tcp"}]',
+        "container_gateway": "172.17.0.1",
+        "container_ip": "172.17.0.9",
+    }
+    assert api.tunnel_target(instance, 3000) == ("172.17.0.9", 3000)
+
+
+def test_tunnel_target_is_empty_without_a_route_to_the_port():
+    instance = {"ports_json": "[]", "container_gateway": "172.17.0.1"}
+    assert api.tunnel_target(instance, 3000) == (None, None)
+    assert api.tunnel_target({"container_ip": "172.17.0.9"}, 0) == (None, None)