fix(images): cache blob avatars by durable blob id, not the ephemeral URL

[bmc08gt]

Problem

Blob avatars that were already loaded get re-downloaded — you see the BlurHash placeholder, then the image, every time — even for images the app has fetched before.

Root cause: a blob's download_url is ephemeral. The proto (blob/v1/model.proto) documents it as "Ephemeral, server-minted URL… Re-issued on every fetch" with an expiry, while the bytes it addresses are immutable. ProfileAvatar keyed Coil's memory and disk caches on that rotating URL (.memoryCacheKey(photoUri) / .data(url)), so every new gRPC response minted a new URL → a new cache key → a guaranteed miss → BlurHash flash + re-download, both within a session and across cold starts.

Fix

Key the caches on the durable blob id instead of the URL, and treat cached blobs as immutable.

• MediaItem gains cacheKeyForSize() / cacheKeyBelow(), derived from the rendition's blobId (base58) — a stable identity that survives URL rotation and app restarts.
• ProfileAvatar sets both memoryCacheKey and diskCacheKey to that stable key. .data() still carries the current URL so the actual network fetch works; only the cache identity changes. The placeholderMemoryCacheKey upgrade (smaller rendition → larger) is keyed the same way so it keeps working across surfaces.
• ImmutableBlobCacheStrategy (new) is wired into the ImageLoader. Coil's default strategy is a stock HTTP cache that would revalidate/re-fetch based on the expiring URL's headers; since blobs are static, this serves a cached response unconditionally and always persists on write.

Result

• Within a session: re-displaying an avatar (list → detail, re-entering a screen, a rotated URL) hits the memory cache under the blob-id key — no BlurHash flash.
• Across cold starts: the disk entry is keyed by blob id and served without revalidation, so previously-loaded images appear immediately instead of re-downloading.

Blobs are static, so no eviction logic is needed beyond the existing 50 MB disk LRU cap.

Notes / scope

• The only wired-up blob image site today is the profile/contact avatar. Chat message media (MessageBubble) is still a TODO and not rendered; when it lands it should reuse MediaItem.cacheKeyForSize() the same way.

Testing

• :services:flipcash, :apps:flipcash:shared:common-ui, :apps:flipcash:app compile clean.
• Manual: load a screen with avatars, force-stop the app, relaunch — avatars should render without the BlurHash-then-image flash.