fix: always set Secure/HttpOnly/SameSite on session cookies (closes #47)

Resolve the two remaining gosec G124 findings (internal/session/
session.go:84 and :128): session cookies are now unconditionally
Secure, HttpOnly, and SameSite=Strict on both the CreateSession
set-cookie path and the ClearSession delete-cookie path. gosec requires
these attributes to be constant, and there is no legitimate
configuration in which the authentication cookie should be weaker, so
the former secure toggle (wired to !config.Debug) is removed rather
than kept as a variable.

The toggle parameter on NewManager is retained as an ignored blank
parameter so existing call sites (including tests) keep compiling;
removing it is tracked as a Future Step in TODO.md. Local development
over http://localhost keeps working because browsers treat localhost as
a trustworthy origin and accept Secure cookies there.

Update TODO.md per its Workflow section: record this step as completed,
promote the manual auth/URL-flow test pass to Next Step, and correct
the stale Status text (make check is now green).

TODO.md

@@ -10,22 +10,27 @@
 
 # Status
 
-pre-1.0. No git tags exist. main (6b4a1d7, 2026-04-07) is current with
-origin; recent work extracted the internal/magic and internal/allowlist
-packages. The 10 gosec findings from the 2026-07-06 morning survey are
-NOT fixed on main: the newest gosec/lint fix commits are from
-2026-02-25 (85729d9, ce6db76) and nothing since touches gosec, so those
-findings remain open.
+pre-1.0. No git tags exist. Recent work extracted the internal/magic,
+internal/allowlist, internal/httpfetcher, and internal/signature
+packages. The gosec findings from the 2026-07-06 survey are resolved:
+the last two open findings (G124, session cookie attributes in
+internal/session) are fixed as of this change, so `make check` is green
+on main.
 
 # Next Step
 
-Fix the 10 open gosec lint findings so make check passes on main again
-(main must always be green; no gosec fix commits have landed since
-2026-02-25). No blanket suppressions; fix or justify each finding
-individually.
+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
 
 # Completed Steps
 
+- 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;
+  `make check` green (closes #47)
 - 2026-07-07 Adopted scripts-to-rule-them-all: `script/` entrypoints,
   Makefile shims, README Entrypoints section
 - 2026-04-07 extract magic byte detection into internal/magic (#42)
@@ -48,14 +53,12 @@ individually.
 
 # Future Steps
 
-- 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
 - P0: validate configuration on startup, fail fast on bad config
+- P1: remove the ignored former secure toggle parameter from
+  session.NewManager and update its call sites (requires touching
+  existing tests; needs approval per repo rules)
 - P1: implement blocked networks configuration to extend SSRF
   protection
 - P1: rate limit global concurrent upstream fetches to prevent