docs: record manual test pass of auth and encrypted URL flows (closes #49)

All six checks from issue #49 executed against a locally built and
running pixad (main at 6573b9d, port 18099, throwaway local config):
login form, wrong-key error, generator form after login, encrypted URL
serving an image, expired URL returning 410, and logout redirect. All
passed, as did the nine checks in scripts/manual-test.sh. Evidence
recorded as a checklist in TODO.md per its Workflow section; the cache
size management and eviction item is promoted to Next Step.

TODO.md

@@ -19,14 +19,34 @@ on main.
 
 # Next Step
 
-P0: manual test pass of the auth and encrypted URL flows, then commit
-the checked-off results to TODO.md: visit / and see the login form;
-wrong key shows an error; correct signing key shows the generator form;
-a generated encrypted URL serves the image; an expired URL (short TTL)
-returns 410; logout redirects back to login
+P0: implement cache size management and eviction so the disk cannot
+fill up
 
 # Completed Steps
 
+- 2026-08-07 manual test pass of the auth and encrypted URL flows
+  against a locally built and running `pixad` (built from `main` at
+  `6573b9d`, port 18099, local throwaway config); all six checks
+  passed, plus all nine tests in `scripts/manual-test.sh` (closes #49):
+    - [x] visit `/` and see the login form: HTTP 200, `Pixa - Login`
+      page with `name="key"` password form
+    - [x] wrong key shows an error: POST `/` with `key=wrong-key`
+      returned HTTP 200 login page containing "Invalid signing key"
+    - [x] correct signing key shows the generator form: POST `/`
+      returned HTTP 303 to `/` with
+      `Set-Cookie: pixa_session=...; HttpOnly; Secure; SameSite=Strict`;
+      GET `/` with that cookie rendered `Pixa - URL Generator` with the
+      `/generate` form and logout link
+    - [x] a generated encrypted URL serves the image: POST `/generate`
+      (ttl=3600) produced a `/v1/e/<token>/img.jpeg` URL that returned
+      HTTP 200, `Content-Type: image/jpeg`, an 800x600 baseline JPEG of
+      61706 bytes
+    - [x] an expired URL (short TTL) returns 410: a ttl=1 URL fetched
+      after 3 s returned HTTP 410 Gone with
+      `{"error":"URL has expired","status":410,...}`
+    - [x] logout redirects back to login: GET `/logout` returned HTTP
+      303 to `/` with `Set-Cookie: pixa_session=; Max-Age=0`;
+      subsequent GET `/` rendered the login form again
 - 2026-08-07 fix the two remaining gosec findings (G124 in
   internal/session): session cookies now always carry
   Secure/HttpOnly/SameSite=Strict on both the set and clear paths;
@@ -53,8 +73,6 @@ returns 410; logout redirects back to login
 
 # Future Steps
 
-- P0: implement cache size management and eviction so the disk cannot
-  fill up
 - P0: validate configuration on startup, fail fast on bad config
 - P1: implement blocked networks configuration to extend SSRF
   protection