Rebrand MMTL to MeBox (name, logo, Docker image) (#17)

* Rebrand MMTL to MeBox across codebase and assets

Rename the project display name, Go module path, environment variable
prefix (MEBOX_*), Docker image references, and UI branding from MMTL/mmtl
to MeBox/mebox. Replace logo assets with the new MeBox icon and keep
legacy SQLite migration support for existing mmtl.db deployments.

Co-authored-by: truewhile <truewhile@users.noreply.github.com>

* Fix logo icons: use cube-only crop without truncated text

Previous icon generation cropped too much of the source image, including
partial MeBox wordmark text that was cut off in square icon containers.
Regenerate logo-64/192/512, favicon, and SVG from cube-only region.

Co-authored-by: truewhile <truewhile@users.noreply.github.com>

---------

Co-authored-by: Cursor Agent <cursoragent@cursor.com>
Co-authored-by: truewhile <truewhile@users.noreply.github.com>

internal/config/normalize.go

@@ -15,7 +15,7 @@ func (c *Config) normalize() error {
 		c.App.DataDir = "./data"
 	}
 	if c.Database.DBPath == "" {
-		c.Database.DBPath = filepath.Join(c.App.DataDir, "mmtl.db")
+		c.Database.DBPath = filepath.Join(c.App.DataDir, "mebox.db")
 	}
 	if c.Database.Type == "" {
 		c.Database.Type = "auto"
@@ -48,14 +48,14 @@ func (c *Config) normalize() error {
 		c.Cache.ImagesMaxSizeMB = 0
 	}
 	if c.Cache.RedisPrefix == "" {
-		c.Cache.RedisPrefix = "mmtl"
+		c.Cache.RedisPrefix = "mebox"
 	}
 	if c.Cache.MediaTTLSeconds < 1 {
 		c.Cache.MediaTTLSeconds = 15
 	}
 	c.Search.Backend = strings.ToLower(strings.TrimSpace(c.Search.Backend))
 	if c.Search.Index == "" {
-		c.Search.Index = "mmtl_media"
+		c.Search.Index = "mebox_media"
 	}
 	if c.Secrets.JWTSecret == "" {
 		// 持久化自动生成的密钥以在操作员忘记配置时保持会话稳定。