Add FastAPI backend and React frontend subprojects

Backend:
- FastAPI app with 17 REST endpoints covering dashboard, monitoring,
  media index, file browser, and jobs
- Reuses existing clients/domain/services unchanged
- pydantic-settings config, dependency injection, CORS setup
- Auto-generated OpenAPI docs at /docs

Frontend:
- Vite + React + TypeScript SPA
- @tanstack/react-query for data fetching with polling
- ag-grid-react for media table and file browser
- recharts for monitoring charts
- Tailwind CSS styling
- 4 pages: Dashboard, Monitoring, Media, File Browser
- Typed API client matching all backend endpoints

Also:
- docs/MIGRATION_PLAN.md with full architecture plan
- Updated .gitignore for both subprojects
- Streamlit app preserved for now (can coexist)

backend/jobs.py

@@ -0,0 +1,59 @@
+"""Template-based remote jobs.
+
+Remote jobs are intentionally explicit templates instead of free-form shell input.
+This keeps the UI safer and makes future destructive operations easier to wrap in
+confirmations/dry-runs.
+"""
+
+from __future__ import annotations
+
+import shlex
+from dataclasses import dataclass
+from typing import Mapping
+
+from clients.ssh import CommandResult, RemoteSSHClient
+
+
+@dataclass(frozen=True)
+class JobTemplate:
+    """Description and command template for one remote job."""
+
+    name: str
+    description: str
+    command_template: str
+    destructive: bool = False
+
+    def render(self, values: Mapping[str, str]) -> str:
+        """Render the command with shell-quoted template values.
+
+        This is what keeps paths with spaces safe when inserted into job commands.
+        """
+        safe_values = {key: shlex.quote(value) for key, value in values.items()}
+        return self.command_template.format(**safe_values)
+
+
+# Phase 1 jobs are intentionally conservative. Add your own templates here later.
+JOB_TEMPLATES: dict[str, JobTemplate] = {
+    "disk_usage": JobTemplate(
+        name="Disk usage for selected path",
+        description="Runs du -sh on the selected remote path.",
+        command_template="du -sh {path}",
+    ),
+    "ffprobe": JobTemplate(
+        name="ffprobe JSON",
+        description="Prints raw ffprobe stream/format metadata.",
+        command_template="ffprobe -v error -show_format -show_streams -print_format json {path}",
+    ),
+    "dry_run_find_empty_dirs": JobTemplate(
+        name="Find empty directories dry-run",
+        description="Lists empty directories under the selected path. Does not delete anything.",
+        command_template="find {path} -type d -empty -print",
+    ),
+}
+
+
+def run_job(ssh: RemoteSSHClient, job_key: str, path: str, timeout: int = 600) -> CommandResult:
+    """Render and execute a configured job template for a selected remote path."""
+    template = JOB_TEMPLATES[job_key]
+    command = template.render({"path": path})
+    return ssh.run(command, timeout=timeout)