feat: Add avatar stacks (#37594)

Parse `Co-authored-by:` trailers from commit messages and surface
contributors as an avatar stack across the commit page, commits list, PR
commits tab, latest-commit row, blame, graph, and dashboard feed.

- Up to 10 visible 20px avatars, GitHub-style overlap (6px first stride,
4px between subsequent), `+N` chip for the rest.
- Label: 1 → name; 2 → `<a> and <b>`; 3+ → `<N> people` opens a Tippy
popup with all participants.
- Names and avatars link to the repo's commits-by-author search; fall
back to profile or `mailto:`.
- Trailer parsing uses `net/mail.ParseAddress`, scans only the trailing
paragraph, filters out the commit's own author/committer.
- Drops the non-standard `Co-committed-by:` emission on squash merge and
web edits.

Devtest: `/devtest/coauthor-avatars`.

Fixes #25521

----
<img width="353" height="277" alt="image"
src="https://github.com/user-attachments/assets/72092ceb-97ca-4b09-9557-0b72d3c5458e"
/>

<img width="533" height="328"
src="https://github.com/user-attachments/assets/11d0c8f8-8b3f-4f2e-9993-879f1c06bcc5"
/>

---------

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Giteabot <teabot@gitea.io>

services/repository/gitgraph/graph_models.go

@@ -13,8 +13,10 @@ import (
 	asymkey_model "gitea.dev/models/asymkey"
 	"gitea.dev/models/db"
 	git_model "gitea.dev/models/git"
+	"gitea.dev/models/gituser"
 	repo_model "gitea.dev/models/repo"
 	user_model "gitea.dev/models/user"
+	"gitea.dev/modules/container"
 	"gitea.dev/modules/git"
 	"gitea.dev/modules/log"
 	asymkey_service "gitea.dev/services/asymkey"
@@ -93,9 +95,7 @@ func (graph *Graph) AddCommit(row, column int, flowID int64, data []byte) error
 // before finally retrieving the latest status
 func (graph *Graph) LoadAndProcessCommits(ctx context.Context, repository *repo_model.Repository, gitRepo *git.Repository) error {
 	var err error
-	var ok bool
-
-	emails := map[string]*user_model.User{}
+	emailSet := make(container.Set[string])
 	keyMap := map[string]bool{}
 
 	for _, c := range graph.Commits {
@@ -106,14 +106,26 @@ func (graph *Graph) LoadAndProcessCommits(ctx context.Context, repository *repo_
 		if err != nil {
 			return fmt.Errorf("GetCommit: %s Error: %w", c.Rev, err)
 		}
-
 		if c.Commit.Author != nil {
-			email := c.Commit.Author.Email
-			if c.User, ok = emails[email]; !ok {
-				c.User, _ = user_model.GetUserByEmail(ctx, email)
-				emails[email] = c.User
-			}
+			emailSet.Add(c.Commit.Author.Email)
 		}
+		for _, sig := range c.Commit.AllParticipantIdentities() {
+			emailSet.Add(sig.Email)
+		}
+	}
+
+	emailUserMap, err := user_model.GetUsersByEmails(ctx, emailSet.Values())
+	if err != nil {
+		log.Error("GetUsersByEmails: %v", err)
+	}
+
+	for _, c := range graph.Commits {
+		if c.Commit == nil {
+			continue
+		}
+
+		c.User = emailUserMap.GetByEmail(c.Commit.Author.Email)
+		c.AvatarStackData = gituser.BuildAvatarStackData(ctx, c.Commit.AllParticipantIdentities(), emailUserMap)
 
 		c.Verification = asymkey_service.ParseCommitWithSignature(ctx, c.Commit)
 
@@ -246,18 +258,19 @@ func newRefsFromRefNames(refNames []byte) []git.Reference {
 
 // Commit represents a commit at coordinate X, Y with the data
 type Commit struct {
-	Commit       *git.Commit
-	User         *user_model.User
-	Verification *asymkey_model.CommitVerification
-	Status       *git_model.CommitStatus
-	Flow         int64
-	Row          int
-	Column       int
-	Refs         []git.Reference
-	Rev          string
-	Date         time.Time
-	ShortRev     string
-	Subject      string
+	Commit          *git.Commit
+	User            *user_model.User // author
+	AvatarStackData *gituser.AvatarStackData
+	Verification    *asymkey_model.CommitVerification
+	Status          *git_model.CommitStatus
+	Flow            int64
+	Row             int
+	Column          int
+	Refs            []git.Reference
+	Rev             string
+	Date            time.Time // author date from "%ad"
+	ShortRev        string
+	Subject         string
 }
 
 // OnlyRelation returns whether this a relation only commit