sidecar: declarative USER_BASEPATHS reconciler

PhotoPrism's OSS edition has no way to map OIDC claims to BasePath, so
every freshly-registered OIDC user lands with BasePath="" and either
sees the whole library (admin) or nothing (guest) — never their own
subfolder.

Introduces a sidecar-driven reconciler with a single env knob the
admin sets in docker-compose / .env.photoprism:

  USER_BASEPATHS="test:test, alice:family/alice, bob:bob"

(`user:originals-relative-path` pairs, comma-separated.) On boot and
every 60s thereafter the sidecar:
  - mkdir -p's the target subdirectory under ORIGINALS_ROOT so
    PhotoPrism's path: ACL filter has somewhere real to point;
  - UPDATEs photoprism.auth_users.base_path for the matching row
    where it differs (idempotent, missing users skipped — they
    materialise on first OIDC login and the next pass catches them).

The reconciler uses a separate gorm connection scoped to the
`photoprism` schema with PhotoPrism's own DB user, since the existing
`sidecar` user only has grants on `mule_sidecar.*`. Connection stays
dormant when PP_DB_PASSWORD is empty — the feature is opt-in via env.

Compose changes: thread PP_DB_* + USER_BASEPATHS through to the
sidecar service. New users.go file isolates the reconciler logic;
main.go calls startUserBasepathReconciler() during boot.

sidecar/users.go

@@ -0,0 +1,124 @@
+package main
+
+import (
+	"fmt"
+	"log/slog"
+	"os"
+	"path/filepath"
+	"strings"
+	"time"
+
+	"gorm.io/driver/mysql"
+	"gorm.io/gorm"
+	"gorm.io/gorm/logger"
+)
+
+// PhotoPrism's open-source edition doesn't expose any way to map OIDC
+// claims to a per-user BasePath — every newly-registered OIDC user
+// lands with `BasePath = ""`, which means PhotoPrism's ACL filter
+// shows them nothing (non-admin) or everything (admin). Neither is the
+// per-user library scope a homelab admin typically wants.
+//
+// This reconciler reads a declarative `USER_BASEPATHS` env at sidecar
+// startup, formatted as `username:originals-relative-path` pairs
+// separated by commas (whitespace tolerated), e.g.:
+//
+//	USER_BASEPATHS="test:test, alice:family/alice, bob:bob"
+//
+// For every entry the sidecar:
+//  1. Ensures the originals subdirectory exists (so PhotoPrism's path:
+//     filter has somewhere to point — empty dirs are fine).
+//  2. UPDATEs `photoprism.auth_users.base_path` for the matching user
+//     row if the current value differs. Idempotent: rows already
+//     matching are skipped, and missing users are no-ops (they'll
+//     materialise when they log in via OIDC; the periodic ticker
+//     catches them on the next pass).
+//
+// A 60-second ticker keeps the mapping in lockstep with new OIDC
+// registrations without needing a webhook.
+
+func parseUserBasepaths(raw string) map[string]string {
+	out := map[string]string{}
+	for _, p := range strings.Split(raw, ",") {
+		p = strings.TrimSpace(p)
+		if p == "" {
+			continue
+		}
+		bits := strings.SplitN(p, ":", 2)
+		if len(bits) != 2 {
+			continue
+		}
+		u := strings.TrimSpace(bits[0])
+		// Strip any leading/trailing slash so the value lands in
+		// auth_users.base_path the same way PhotoPrism's own user-edit
+		// UI persists it (relative, no slashes).
+		path := strings.Trim(strings.TrimSpace(bits[1]), "/")
+		if u == "" || path == "" {
+			continue
+		}
+		out[u] = path
+	}
+	return out
+}
+
+func reconcileUserBasepaths(ppDSN, originalsRoot string, mapping map[string]string) error {
+	if len(mapping) == 0 {
+		return nil
+	}
+	if ppDSN == "" {
+		return fmt.Errorf("USER_BASEPATHS set but PP_DB_PASSWORD missing — can't reach photoprism schema")
+	}
+	db, err := gorm.Open(mysql.Open(ppDSN), &gorm.Config{
+		Logger: logger.Default.LogMode(logger.Warn),
+	})
+	if err != nil {
+		return fmt.Errorf("open pp db: %w", err)
+	}
+	if sqlDB, derr := db.DB(); derr == nil {
+		defer sqlDB.Close()
+	}
+
+	for username, path := range mapping {
+		target := filepath.Join(originalsRoot, path)
+		if err := os.MkdirAll(target, 0o775); err != nil {
+			slog.Warn("user-basepath: mkdir failed", "user", username, "path", target, "err", err)
+			// Continue — the DB update is still useful so PhotoPrism's
+			// ACL kicks in even if the directory is created later.
+		}
+
+		res := db.Exec(`UPDATE auth_users
+			SET base_path = ?
+			WHERE user_name = ?
+			  AND COALESCE(base_path, '') <> ?
+			  AND deleted_at IS NULL`,
+			path, username, path)
+		if res.Error != nil {
+			slog.Error("user-basepath: update failed", "user", username, "err", res.Error)
+			continue
+		}
+		if res.RowsAffected > 0 {
+			slog.Info("user-basepath: set", "user", username, "path", path)
+		}
+	}
+	return nil
+}
+
+func startUserBasepathReconciler(cfg *Config) {
+	if len(cfg.UserBasepaths) == 0 {
+		return
+	}
+	slog.Info("user-basepath: starting reconciler", "entries", len(cfg.UserBasepaths))
+	apply := func() {
+		if err := reconcileUserBasepaths(cfg.PpDSN, cfg.OriginalsRoot, cfg.UserBasepaths); err != nil {
+			slog.Warn("user-basepath: reconciliation error", "err", err)
+		}
+	}
+	apply()
+	go func() {
+		t := time.NewTicker(60 * time.Second)
+		defer t.Stop()
+		for range t.C {
+			apply()
+		}
+	}()
+}