Harden filter parser/lexer against malicious search expressions (DoS)

[MrEchoFi]

Fixes #153 .

filter.ParseQuery is reachable from the unauthenticated GraphQL API via
search/filter fields (setHttpRequestLogFilter, setSenderRequestFilter,
updateInterceptSettings). This PR fixes two denial-of-service issues in
pkg/filter.

1. Stack overflow → whole-process crash (critical)

The recursive-descent parser had no depth limit. Deeply nested input — many
( or repeated NOT — recurses through parseExpression until the goroutine
stack is exhausted. In Go a stack overflow is a fatal runtime error: it
cannot be caught by recover(), so gqlgen's panic-recovery middleware does
not contain it and the entire process crashes (proxy, admin API and all
in-flight intercepts). The parse happens before the "no active project" check,
so no project/auth/setup is required.

Fix: add a maxParseDepth (256) guard in parseExpression, the single
recursion point reached by grouped/prefix/infix sub-expression parsing.
Legitimate, human-written queries nest far below this.

2. Lexer goroutine leak (high)

NewLexer runs a goroutine that emits tokens on an unbuffered channel. When
ParseQuery returned early on a parse error it stopped draining the channel,
leaving that goroutine blocked on its send forever. Every malformed query
leaked one goroutine → gradual goroutine/memory exhaustion, also remotely
triggerable.

Fix: make the lexer cancellable (a done channel + Stop()); ParseQuery
defers Stop() so the goroutine is released on every return path.

How to verify

Crash on current main:

cat > pkg/filter/poc_test.go <<'EOF'
package filter
import ("strings";"testing")
func TestPoC(t *testing.T){ ParseQuery(strings.Repeat("(", 2_000_000)) }
EOF
go test ./pkg/filter/ -run TestPoC   # => fatal error: stack overflow
rm pkg/filter/poc_test.go

With this PR:

go test ./pkg/filter/ -run 'Depth|Leak|Nesting' -v   # all PASS, no crash
go test ./pkg/filter/                                  # full suite green

Tests added

• TestParseQueryDepthLimit — 1M-deep ( / NOT / nested-group inputs now
  return ErrMaxDepthExceeded instead of crashing.
• TestParseQueryReasonableNestingStillWorks — legitimate nesting still parses.
• Goroutine-leak regression test.

No behavior change for valid expressions; no public API change.