feat(robot): per-game display names from a wide name corpus

Decouple the displayed opponent name from the small pool of durable robot
accounts: the disguised auto-match robot now gets a freshly composed name each
game, stamped on a new game_players.display_name seat snapshot. The snapshot
also captures humans' names, freezing what an opponent sees for the life of a
game (a later rename no longer rewrites past games); readers fall back to the
account's current name for pre-migration rows.

Names come from a wide composed corpus (internal/robot/namevariety.go): Western
locales (EN/DE/ES/IT/FR/PT), native Japanese/Chinese names, a gender-agreed
Russian pool, and human-style handles. Routing keeps Pick's spirit -- a Russian
game draws Cyrillic + <=20% Latin and never a CJK script; an English game the
full corpus -- via robot.PickNamed.

Loosen account.ValidateDisplayName (and the UI mirror) to admit a trailing run
of up to five digits, so "Player2007"-style handles are valid for humans too
and the disguised robot stays indistinguishable.

Migration 00007 adds game_players.display_name (additive, NOT NULL DEFAULT '');
jet regenerated. Docs (ARCHITECTURE 7, FUNCTIONAL + _ru, PLAN, README) updated.

backend/internal/account/profile.go

@@ -23,9 +23,10 @@ import (
 // is unbounded; auto-provisioned platform names bypass this editor validation).
 const maxDisplayName = 32
 
-// maxDisplayNameSpecials caps the total special characters (the "." / "_" separators —
-// every name rune that is neither a letter nor a space) an editable display name may
-// carry, so a still-well-formed name cannot be made of mostly punctuation.
+// maxDisplayNameSpecials caps the total special characters (every name rune that is
+// neither a letter, a space, nor a digit — i.e. the "." / "_" separators) an editable
+// display name may carry, so a still-well-formed name cannot be made of mostly
+// punctuation. A trailing digit run is bounded separately by displayNameRe.
 const maxDisplayNameSpecials = 5
 
 // maxAwayWindow bounds the daily away window's duration (midnight-wrap aware).
@@ -34,9 +35,11 @@ const maxAwayWindow = 12 * time.Hour
 // displayNameRe enforces the editable display-name format: Unicode letters
 // joined by single space / "." / "_" separators, where a "." or "_" may be followed
 // by a single space. No leading separator and no two adjacent separators (except
-// "<dot|underscore> <space>"); a single trailing "." is allowed, so
-// "Name_P. Last" and "Anna B." are valid, "Name P._Last" is not.
-var displayNameRe = regexp.MustCompile(`^\p{L}+(?:(?:[._] ?| )\p{L}+)*\.?$`)
+// "<dot|underscore> <space>"). The name may end with EITHER a single trailing "."
+// (an initial, "Anna B.") OR a run of 1–5 digits (a handle's number or year,
+// "Player2007"), but not both; digits never appear elsewhere. So "Name_P. Last",
+// "Anna B." and "Аня2007" are valid, while "Name P._Last" and "Dark2Wolf" are not.
+var displayNameRe = regexp.MustCompile(`^\p{L}+(?:(?:[._] ?| )\p{L}+)*(?:\.|[0-9]{1,5})?$`)
 
 // ErrInvalidProfile is returned when a profile update carries an unacceptable
 // field (an unknown language, an invalid timezone, or an over-long display name).
@@ -117,7 +120,7 @@ func ValidateDisplayName(raw string) (string, error) {
 	}
 	specials := 0
 	for _, r := range name {
-		if r != ' ' && !unicode.IsLetter(r) {
+		if r != ' ' && !unicode.IsLetter(r) && !unicode.IsDigit(r) {
 			specials++
 		}
 	}