Test logging in, logging out, the URL generator and /v1/e/ tokens (closes #77)

New handler tests in internal/handlers, with no network: GET / shows the
login form without a session; a wrong key shows it again with an error and
sets no session cookie; the right key answers 303 with a session cookie
marked Secure, HttpOnly and SameSite=Strict, with which GET / shows the
generator page; GET /logout empties the cookie with Max-Age=0; POST
/generate without a session answers 303 to /; /v1/e/ serves a valid
token's image, answers 410 for an expired token and 400 for one changed,
cut short or made with another signing key; a URL made on the generator
page is served by /v1/e/. No code changes.

Model: opus-5-5

TODO.md

@@ -29,6 +29,17 @@ P2: security: referer blacklist
 
 # Completed Steps
 
+- 2026-10-04 logging in, logging out, the URL generator and `/v1/e/` have
+  handler tests (closes #77): new tests in `internal/handlers`, with no
+  network, check that `GET /` without a login session shows the login form; a
+  wrong key shows it again with an error and sets no session cookie; the right
+  key answers 303 to `/` with a session cookie marked `Secure`, `HttpOnly` and
+  `SameSite=Strict`, with which `GET /` shows the generator page; `GET /logout`
+  answers 303 to `/` with an empty session cookie sent with `Max-Age=0`;
+  `POST /generate` without a login session answers 303 to `/`; `/v1/e/` serves
+  the image for a valid token, answers 410 for an expired one and 400 for one
+  with a character changed, cut short or made with another signing key; and a
+  URL made on the generator page is served by `/v1/e/`. No code changes.
 - 2026-10-04 `TestEvictionRunsOnPeriodicSchedule` no longer races the evictor
   (closes #183): it wrote each variant file and then inserted its accounting row
   by hand, and a reconciliation pass between the two adopted the file first, so