Correct doc and help sentences that are false about the code (closes #233)

A blob is written in full to a temporary file in $TMPDIR and uploaded
once finished, not streamed to storage; the README and
config.example.yml now say a backup needs about blob_size_limit of free
space there. Also corrected: the snapshot ID format, what restore reads
and how incomplete snapshots are removed in docs/DATAMODEL.md, what
source_path is for, the index_path default, the config search order in
the snapshot create help, what snapshot remove cleans up in the prune
help, how the release installs Go, and the script/release and
script/fmt-check comments.

The source_path claim is also corrected in models.go, scanner.go and
001.sql, which the issue did not list.

Model: opus-5-5

ARCHITECTURE.md

@@ -54,7 +54,7 @@ The database tracks five primary entities and their relationships:
 
 #### File (`database.File`)
 Represents a file, directory, or symlink in the backup system. Stores metadata needed for restoration:
-- Path, source_path (for restore path stripping), mtime
+- Path, source_path (the configured snapshot path the scan found it under), mtime
 - Size, mode, ownership (uid, gid)
 - Symlink target (if applicable)
 
@@ -82,9 +82,9 @@ The final storage unit uploaded to S3. Contains many compressed and encrypted ch
 Blob creation process:
 1. Chunks are accumulated (up to MaxBlobSize, typically 10GB)
 2. As each chunk is added, its uncompressed bytes are fed to a running SHA-256
-3. Concurrently, the same bytes are compressed with zstd, then encrypted with age (recipients configured in config), and streamed to storage
+3. Concurrently, the same bytes are compressed with zstd, then encrypted with age (recipients configured in config), and written to a temporary file in `$TMPDIR` (`/tmp` when unset)
 4. On finalize, the blob's name is the double SHA-256 of the uncompressed contents — `hex(SHA256(SHA256(...)))` — not a hash of the compressed, encrypted bytes
-5. Uploaded to `blobs/{hash[0:2]}/{hash[2:4]}/{hash}`
+5. The finished file is uploaded to `blobs/{hash[0:2]}/{hash[2:4]}/{hash}` and then deleted, so a backup needs about `blob_size_limit` of free space in `$TMPDIR`
 
 #### BlobChunk (`database.BlobChunk`)
 Maps chunks to their position within blobs: