perf(indexer): éliminer le pre-count WalkDir en mode incrémental + concurrence adaptative

- Incremental rebuild: remplace le WalkDir de comptage par un COUNT(*) SQL
  → incrémental 67s → 25s (-62%) sur disque externe
- Full rebuild: conserve le WalkDir (DB vidée avant le comptage)
- Concurrence par défaut: num_cpus/2 clampé [2,8] au lieu de 2 fixe
- Ajoute num_cpus comme dépendance workspace
- Backoffice jobs: un seul formulaire avec formAction par bouton (icônes rétablies)
- infra/perf.sh: corrige l'endpoint /index/jobs/:id (pas /details), exporte BASE_API/TOKEN

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Cargo.lock

@@ -863,6 +863,12 @@ version = "0.5.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "2304e00983f87ffb38b55b444b5e3b60a884b5d30c0fca7d82fe33449bbe55ea"
 
+[[package]]
+name = "hermit-abi"
+version = "0.5.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "fc0fef456e4baa96da950455cd02c081ca953b141298e41db3fc7e36b1da849c"
+
 [[package]]
 name = "hex"
 version = "0.4.3"
@@ -1171,6 +1177,7 @@ dependencies = [
  "futures",
  "image",
  "notify",
+ "num_cpus",
  "parsers",
  "rand 0.8.5",
  "rayon",
@@ -1639,6 +1646,16 @@ dependencies = [
  "libm",
 ]
 
+[[package]]
+name = "num_cpus"
+version = "1.17.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "91df4bbde75afed763b708b7eee1e8e7651e02d97f6d5dd763e89367e957b23b"
+dependencies = [
+ "hermit-abi",
+ "libc",
+]
+
 [[package]]
 name = "once_cell"
 version = "1.21.3"