feat(version): version x.y.z propre et cohérente partout + bump script

Corrige l'incoherence d'affichage (page principale 0.0.0-dev vs a propos
v2.0.0-dev) et unifie la gestion de version sur une seule source de verite.

Backend:
- version.py: get_version() retourne toujours le tag x.y.z propre du dernier
  tag (plus de 0.0.0-dev / -N-gHASH dans l'UI). Ajout get_git_describe() et
  get_git_commit() pour le detail.
- main.py: /api/health expose git_describe + git_commit (nouveaux champs).

Frontend:
- modale A propos (populateAbout): version + commit lus depuis /api/health,
  suppression du hardcode v2.0.0-dev dans index.html.
- section config A propos (loadAbout): lit health.version, plus de state.
  APP_VERSION obsolete.
- badge header: fetch /api/health (fallback neutre '...').
- state.js: suppression de APP_VERSION (1.5.0) devenu mort; imports nettoyes.

Script:
- scripts/bump_version.sh: incrementation SemVer selon les commits
  (breaking->major, feat->minor, sinon patch), creer/pousser le tag vX.Y.Z.

Tests: 507 passed, frontend validators OK.

backend/main.py

@@ -287,9 +287,11 @@ class ReloadResponse(BaseModel):
 class HealthResponse(BaseModel):
     """Application health status."""
     status: str = Field(description="Health status ('ok' or 'error')")
-    version: str = Field(description="Application version")
+    version: str = Field(description="Application version (x.y.z — latest release tag)")
     vaults: int = Field(description="Number of configured vaults")
     total_files: int = Field(description="Total indexed files across all vaults")
+    git_describe: str = Field(default="", description="Full git describe string (commits beyond tag), empty if no git")
+    git_commit: str = Field(default="", description="Short HEAD commit hash, empty if no git")
 
 
 class DirectoryCreateRequest(BaseModel):
@@ -645,7 +647,7 @@ async def lifespan(app: FastAPI):
     _search_executor = None
 
 
-from backend.version import get_version
+from backend.version import get_version, get_git_describe, get_git_commit
 
 app = FastAPI(title="ObsiGate", version=get_version(), lifespan=lifespan)
 
@@ -1029,6 +1031,8 @@ async def api_health():
         "version": app.version,
         "vaults": len(index),
         "total_files": total_files,
+        "git_describe": get_git_describe(),
+        "git_commit": get_git_commit(),
     }