[feature] Add S3 key prefix (#4200)

Been running these changes on on my live instance without any issues as far as I can tell. It's been playing nice with multiple instances in the same bucket.

# Description

This lets users prefix their object storage files.
Useful for when you want to host multiple GTS instances inside
the same bucket. Providers like Backblaze limit the number of buckets
you can have on your account so grouping by prefix may be more desirable
in this situation.

closes #1371

## Checklist

Please put an x inside each checkbox to indicate that you've read and followed it: `[ ]` -> `[x]`

If this is a documentation change, only the first checkbox must be filled (you can delete the others if you want).

- [x] I/we have read the [GoToSocial contribution guidelines](https://codeberg.org/superseriousbusiness/gotosocial/src/branch/main/CONTRIBUTING.md).
- [x] I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
- [x] I/we have not leveraged AI to create the proposed changes.
- [x] I/we have performed a self-review of added code.
- [x] I/we have written code that is legible and maintainable by others.
- [x] I/we have commented the added code, particularly in hard-to-understand areas.
- [x] I/we have made any necessary changes to documentation.
- [ ] I/we have added tests that cover new code.
- [ ] I/we have run tests and they pass locally with the changes.
- [x] I/we have run `go fmt ./...` and `golangci-lint run`.

Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4200
Co-authored-by: vdyotte <vdyotte@gmail.com>
Co-committed-by: vdyotte <vdyotte@gmail.com>

internal/config/helpers.gen.go

@@ -104,6 +104,7 @@ func (cfg *Configuration) RegisterFlags(flags *pflag.FlagSet) {
 	flags.Bool("storage-s3-proxy", cfg.StorageS3Proxy, "Proxy S3 contents through GoToSocial instead of redirecting to a presigned URL")
 	flags.String("storage-s3-redirect-url", cfg.StorageS3RedirectURL, "Custom URL to use for redirecting S3 media links. If set, this will be used instead of the S3 bucket URL.")
 	flags.String("storage-s3-bucket-lookup", cfg.StorageS3BucketLookup, "S3 bucket lookup type to use. Can be 'auto', 'dns' or 'path'. Defaults to 'auto'.")
+	flags.String("storage-s3-key-prefix", cfg.StorageS3KeyPrefix, "Prefix to use for S3 keys. This is useful for separating multiple instances sharing the same S3 bucket.")
 	flags.Int("statuses-max-chars", cfg.StatusesMaxChars, "Max permitted characters for posted statuses, including content warning")
 	flags.Int("statuses-poll-max-options", cfg.StatusesPollMaxOptions, "Max amount of options permitted on a poll")
 	flags.Int("statuses-poll-option-max-chars", cfg.StatusesPollOptionMaxChars, "Max amount of characters for a poll option")
@@ -285,6 +286,7 @@ func (cfg *Configuration) MarshalMap() map[string]any {
 	cfgmap["storage-s3-proxy"] = cfg.StorageS3Proxy
 	cfgmap["storage-s3-redirect-url"] = cfg.StorageS3RedirectURL
 	cfgmap["storage-s3-bucket-lookup"] = cfg.StorageS3BucketLookup
+	cfgmap["storage-s3-key-prefix"] = cfg.StorageS3KeyPrefix
 	cfgmap["statuses-max-chars"] = cfg.StatusesMaxChars
 	cfgmap["statuses-poll-max-options"] = cfg.StatusesPollMaxOptions
 	cfgmap["statuses-poll-option-max-chars"] = cfg.StatusesPollOptionMaxChars
@@ -1029,6 +1031,14 @@ func (cfg *Configuration) UnmarshalMap(cfgmap map[string]any) error {
 		}
 	}
 
+	if ival, ok := cfgmap["storage-s3-key-prefix"]; ok {
+		var err error
+		cfg.StorageS3KeyPrefix, err = cast.ToStringE(ival)
+		if err != nil {
+			return fmt.Errorf("error casting %#v -> string for 'storage-s3-key-prefix': %w", ival, err)
+		}
+	}
+
 	if ival, ok := cfgmap["statuses-max-chars"]; ok {
 		var err error
 		cfg.StatusesMaxChars, err = cast.ToIntE(ival)
@@ -3793,6 +3803,31 @@ func GetStorageS3BucketLookup() string { return global.GetStorageS3BucketLookup(
 // SetStorageS3BucketLookup safely sets the value for global configuration 'StorageS3BucketLookup' field
 func SetStorageS3BucketLookup(v string) { global.SetStorageS3BucketLookup(v) }
 
+// StorageS3KeyPrefixFlag returns the flag name for the 'StorageS3KeyPrefix' field
+func StorageS3KeyPrefixFlag() string { return "storage-s3-key-prefix" }
+
+// GetStorageS3KeyPrefix safely fetches the Configuration value for state's 'StorageS3KeyPrefix' field
+func (st *ConfigState) GetStorageS3KeyPrefix() (v string) {
+	st.mutex.RLock()
+	v = st.config.StorageS3KeyPrefix
+	st.mutex.RUnlock()
+	return
+}
+
+// SetStorageS3KeyPrefix safely sets the Configuration value for state's 'StorageS3KeyPrefix' field
+func (st *ConfigState) SetStorageS3KeyPrefix(v string) {
+	st.mutex.Lock()
+	defer st.mutex.Unlock()
+	st.config.StorageS3KeyPrefix = v
+	st.reloadToViper()
+}
+
+// GetStorageS3KeyPrefix safely fetches the value for global configuration 'StorageS3KeyPrefix' field
+func GetStorageS3KeyPrefix() string { return global.GetStorageS3KeyPrefix() }
+
+// SetStorageS3KeyPrefix safely sets the value for global configuration 'StorageS3KeyPrefix' field
+func SetStorageS3KeyPrefix(v string) { global.SetStorageS3KeyPrefix(v) }
+
 // StatusesMaxCharsFlag returns the flag name for the 'StatusesMaxChars' field
 func StatusesMaxCharsFlag() string { return "statuses-max-chars" }