fix(#770): treat region 'All' as no-filter + document region behavior (#1026)

## Summary

Fixes #770 — selecting "All" in the region filter dropdown produced an
empty channel list.

## Root cause

`normalizeRegionCodes` (cmd/server/db.go) treated any non-empty input as
a literal IATA code. The frontend region filter labels its catch-all
option **"All"**; while `region-filter.js` normally sends an empty
string when "All" is selected, any code path that ends up sending
`?region=All` (deep-link URLs, manual queries, future callers) caused
the function to return `["ALL"]`. Downstream queries then filtered
observers for `iata = 'ALL'`, which never matches anything → empty
response.

## Fix

`normalizeRegionCodes` now treats `All` / `ALL` / `all`
(case-insensitive, with optional whitespace, mixed in CSV) as equivalent
to an empty value, returning `nil` to signal "no filter". Real IATA
codes (`SJC`, `PDX`, `sjc,PDX` → `[SJC PDX]`) still pass through
unchanged.

This is a defensive server-side fix: a single chokepoint that all
region-aware endpoints already flow through (channels, packets,
analytics, encrypted channels, observer ID resolution).

## Documentation

Expanded `_comment_regions` in `config.example.json` to explain:
- How IATA codes are resolved (payload > topic > source config — set in
#1012)
- What the `regions` map controls (display labels) vs runtime-discovered
codes
- That observers without an IATA tag only appear under "All Regions"
- That the `All` sentinel is server-side safe

## TDD

- **Red commit** (`4f65bf4`): `cmd/server/region_filter_test.go` —
`TestNormalizeRegionCodes_AllIsNoFilter` asserts `All` / `ALL` / `all` /
`""` / `"All,"` all collapse to `nil`. Compiles, runs, fails on
assertion (`got [ALL], want nil`). Companion test
`TestNormalizeRegionCodes_RealCodesPreserved` locks in that `sjc,PDX`
still returns `[SJC PDX]`.
- **Green commit** (`c9fb965`): two-line change in
`normalizeRegionCodes` + docs update.

## Verification

```
$ go test -run TestNormalizeRegionCodes -count=1 ./cmd/server
ok      github.com/corescope/server     0.023s

$ go test -count=1 ./cmd/server
ok      github.com/corescope/server    21.454s
```

Full suite green; no existing region tests regressed.

Fixes #770

---------

Co-authored-by: Kpa-clawbot <bot@corescope>

cmd/server/db.go

@@ -1105,6 +1105,17 @@ func (db *DB) GetObserverIdsForRegion(regionParam string) ([]string, error) {
 	return ids, nil
 }
 
+// normalizeRegionCodes parses a region query parameter into a list of upper-case
+// IATA codes. Returns nil to signal "no filter" (match all regions).
+//
+// Sentinel handling (issue #770): the frontend region filter dropdown labels its
+// catch-all option "All". When that option is selected the UI may send
+// ?region=All; older code interpreted that literally and tried to match an
+// IATA code "ALL", which never exists, returning an empty result set. Treat
+// "All" / "ALL" / "all" (case-insensitive, optionally surrounded by whitespace
+// or mixed with empty CSV slots) as equivalent to an empty value.
+//
+// Real IATA codes (e.g. "SJC", "PDX") still pass through unchanged.
 func normalizeRegionCodes(regionParam string) []string {
 	if regionParam == "" {
 		return nil
@@ -1113,9 +1124,13 @@ func normalizeRegionCodes(regionParam string) []string {
 	codes := make([]string, 0, len(tokens))
 	for _, token := range tokens {
 		code := strings.TrimSpace(strings.ToUpper(token))
-		if code != "" {
-			codes = append(codes, code)
+		if code == "" || code == "ALL" {
+			continue
 		}
+		codes = append(codes, code)
+	}
+	if len(codes) == 0 {
+		return nil
 	}
 	return codes
 }

cmd/server/region_filter_test.go

@@ -0,0 +1,41 @@
+package main
+
+import (
+	"testing"
+)
+
+// Issue #770: the region filter dropdown's "All" option was being sent to the
+// backend as ?region=All. The backend then tried to match observers with IATA
+// code "ALL", which never exists, producing an empty channel/packet list.
+//
+// "All" / "ALL" / "all" / "" must all be treated as "no region filter".
+func TestNormalizeRegionCodes_AllIsNoFilter(t *testing.T) {
+	cases := []struct {
+		name string
+		in   string
+	}{
+		{"empty", ""},
+		{"literal All (frontend dropdown label)", "All"},
+		{"upper ALL", "ALL"},
+		{"lower all", "all"},
+		{"All with whitespace", "  All  "},
+		{"All in csv with empty siblings", "All,"},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			got := normalizeRegionCodes(tc.in)
+			if got != nil {
+				t.Errorf("normalizeRegionCodes(%q) = %v, want nil (no filter)", tc.in, got)
+			}
+		})
+	}
+}
+
+// Real region codes must still pass through unchanged (case-folded to upper).
+// This locks in that the "All" handling does not regress legitimate filters.
+func TestNormalizeRegionCodes_RealCodesPreserved(t *testing.T) {
+	got := normalizeRegionCodes("sjc,PDX")
+	if len(got) != 2 || got[0] != "SJC" || got[1] != "PDX" {
+		t.Errorf("normalizeRegionCodes(\"sjc,PDX\") = %v, want [SJC PDX]", got)
+	}
+}