Bound every slog line against client-chosen text (closes #176)

MaxBodySize logged r.URL.Path untruncated at WARN, and routes.go
registers it ahead of RequireAuth, so an unauthenticated
POST /source/<8 KB>/edit with an oversize declared Content-Length wrote
attacker-chosen text of attacker-chosen length into the operator's log,
for the cost of a request with no body. The 2,560-byte per-line budget
from #146 did not reach it: that budget lives in the access log's field
capping and this is a separate slog call.

The capping mechanism moves out of internal/middleware into
internal/logfield so there is one budget and one implementation rather
than a second ad-hoc truncation. Truncate and EncodedBytes are
unchanged; the access log now spends logfield.MaxBytes where it spent
maxLogFieldBytes.

The sweep the issue asked for found five more call sites of the same
shape, all reachable unauthenticated, all now capped: the CSRF 403
(also registered ahead of RequireAuth), the rate limiters' 429 (the
per-entrypoint receiver limiter is unauthenticated), RequireAuth's own
DEBUG line, the unknown-entrypoint DEBUG line on the receiver, and the
failed-login DEBUG lines. DEBUG being off by default is not a bound: an
operator turning it on to diagnose a flood must not thereby hand the
flood an unbounded write. Every other slog call in the tree was read
and judged; the PR body lists all of them, including the ones left
alone and why.

Two further sites arrived in next with #171 after the first sweep was
written and are capped here as well: "login failure limit exceeded" in
loginguard.go and "password verification capacity exhausted" in
handlers/auth.go, both WARN on the unauthenticated login POST. Neither
was ever wide — chi routes that POST on a static pattern, so r.URL.Path
is the 12-byte constant /pages/login and each line lands near 120
bytes. They are capped because RecordLoginFailure is exported and takes
any *http.Request, so the bound rests on a routing invariant nobody
wrote down, and because the same message at handlers/profile.go logs no
path at all. No request through the mux can widen either line, so their
tests call those two entry points directly with the path a caller on a
parameterised route would supply; that is what the caps defend against,
and an unasserted cap is one a later edit removes for free.

MaxBodySize stays ahead of RequireAuth. An oversize body should be
refused before the request buys a cookie decrypt and a session load,
and rejecting first is what keeps an unauthenticated flood from
choosing how much session work the process does. The ordering and what
it costs are now written at the registration, on maxFormBodySize.

MaxAccessLogLineBytes is restated as the ceiling on every slog line
carrying text an UNAUTHENTICATED client supplies, not just the access
log's: each of these lines carries strictly fewer client-supplied
fields than the access log does, so none can be wider. That is asserted
per line under both handlers rather than argued. The claim is qualified
rather than universal because three kinds of writer are outside it, and
the README and the constant now name all three: lines carrying an
authenticated operator's own input, which are not truncated at all (the
webhook name on "webhook created" reaches 600 KB on one line from a
100 KB form field, measured; the SSRF-rejection url and the target_name
lines are the same shape) and are left uncapped deliberately, since
truncating the operator's own configuration echoed back costs
debuggability against no adversary; the log delivery target, which
exists to emit the whole event; and GORM's default logger, which prints
the interpolated SQL to stdout on a record-not-found and is unbounded
on the receiver and login lookups. That last one is a real defect this
audit turned up and is filed separately as #178, not fixed here.

Tests drive 8 KB of client-chosen text at all eight sites, through both
handlers internal/logger can install and through each character they
escape — including a bare C0 control, which costs six bytes on the line
against the one it cost to send and is the case a raw-byte budget
breaks on first. Each holds the encoded line to the ceiling and asserts
the markers at the far end of the input are absent, so a value that
merely happened to be short cannot pass; for the six sites a request
can widen, the whole flood's output is held to what that ceiling
allows. The two login lines past the username lookup, capped for
uniformity rather than need, are pinned too. internal/logfield gains a
test that measures the per-rune charge against what the handlers really
emit over roughly 3,000 code points on each, so an undercharged rune
fails a test instead of quietly falsifying the ceiling.

Verified by mutation: reverting the MaxBodySize cap alone fails 28
subtests with a 16,583-byte line against the 2,560 ceiling; reverting
the other five fails 70; reverting either login-throttle WARN cap fails
14, through the direct calls those caps exist for; uncapping either of
the two login lines past the username lookup fails both handlers on its
own, so those two are independently pinned rather than jointly;
budgeting raw bytes instead of encoded ones fails 23 across three
packages.

README.md

@@ -1188,26 +1188,11 @@ entry points directly with the path such a caller would supply.
 Removing either cap fails 14 subtests.
 
 `internal/middleware/logbound_test.go` and
-`internal/handlers/logbound_test.go` drive 8 KB of client-chosen text
+`internal/handlers/logbound_test.go` drive 8 KB of client-chosen text at
-at each of these — 1 KB at `invalid password`, whose accounts are
+each of these, through both handlers and through every character the
-shared with the successful-login line, where a username past 4 KB
+handlers escape, and hold each line to the 2,560-byte ceiling — and, for
-overflows the session cookie and answers 500 before that line is
+the six rows a request can widen, the whole flood's output to what that
-written — through both handlers, and through seven fills: plain text
+ceiling allows, which is the property an operator actually cares about.
-as the baseline, and then the quotation mark, backslash, tab, newline,
-C0 control and astral non-printable, six characters the wider of the
-two handlers spends more on than the client spent sending them. Every
-case holds each line to the 2,560-byte ceiling. That per-line ceiling
-is what the figure above states, and every row establishes it.
-
-Three of the sites go further and bound the whole flood's output — the
-total bytes a run of distinct invented values wrote, which is the
-shape an operator sizing storage cares about. They are
-`request body exceeds limit`
-(`TestMaxBodySize_FloodOfOversizePathsDoesNotGrowTheLog`),
-`entrypoint not found` and `user not found` (the last two through
-`assertBoundedFlood`). The other rows carry no aggregate assertion;
-the per-line ceiling is what they establish.
-
 `internal/logfield/logfield_test.go` measures the per-rune charge
 against what the handlers really emit, over roughly 3,000 code points on
 each, so an undercharged rune fails a test rather than quietly