feat(firmware): ELF symbol store for server-side crash decoding

Crash reports carry elf_sha256 (first 16 hex chars of the crashing build's
ELF SHA-256). ELFs are now stored under that key in
{firmware_storage_path}/elf/, uploaded either with a firmware release
(optional elf_file on POST /upload and PUT /{id}) or standalone via
POST /api/firmware/elf. The ELF is validated before the release is created.

GET /api/firmware/elf/{sha} reports whether that build's ELF exists and
whether the decoder is available; POST /api/firmware/elf/{sha}/decode runs
xtensa-esp32-elf-addr2line -pfiaC over the addresses. The decoder is looked
up via settings.addr2line_path or PATH. It is NOT in the Docker image yet,
so decoding reports "not installed" until the toolchain is added; the
console falls back to a copy-paste addr2line command.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/config.py

@@ -35,6 +35,9 @@ class Settings(BaseSettings):
     # Local file storage
     built_melodies_storage_path: str = "./storage/built_melodies"
     firmware_storage_path: str = "./storage/firmware"
+    # xtensa-esp32-elf-addr2line for server-side crash decoding (firmware/elf_store.py).
+    # Empty = look it up on PATH; if absent, the console shows a copy-paste command instead.
+    addr2line_path: str = ""
     flash_assets_storage_path: str = "./storage/flash_assets"
     melody_binaries_storage_path: str = "./storage/melody_binaries"
     melody_download_base_url: str = "http://melodies.bellsystems.net/download"