Add per-profile cache documentation

docs/per-profile-cache.md

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+# Per-profile cache system
+
+This document explains how bitvid caches profile metadata across the app, what
+data is stored, and when it is persisted or refreshed.
+
+## Cache layers
+
+bitvid uses two app-wide caches plus a comment-thread cache:
+
+1. **Profile metadata cache** (`bitvid:profileCache:v1`)
+   - Short-lived metadata cache persisted to `localStorage`.
+   - Owned by `js/state/cache.js`, surfaced via `AuthService`.
+2. **Saved profiles list** (`bitvid:savedProfiles:v1`)
+   - Long-lived list of accounts authenticated in this browser.
+   - Stored in `localStorage` and described in `docs/nostr-auth.md`.
+3. **Comment thread profile cache** (in-memory only)
+   - Per-thread cache inside `js/services/commentThreadService.js`.
+   - Seeds from the app-wide profile cache when available.
+
+## Profile metadata cache (`bitvid:profileCache:v1`)
+
+### Storage and lifetime
+
+- **In-memory:** `profileCache` map in `js/state/cache.js`.
+- **Persistent:** `localStorage` key `bitvid:profileCache:v1`.
+- **TTL:** 10 minutes (`PROFILE_CACHE_TTL_MS`).
+- **Eviction:** Entries are filtered/removed on load, read, and persist when they
+  exceed TTL.
+
+### Stored data (per pubkey)
+
+Entries are normalized before storage and include:
+
+- `name`
+- `picture` (sanitized via `sanitizeProfileMediaUrl`)
+- Optional fields: `about`, `website`, `banner`, `lud16`, `lud06`,
+  `lightningAddress`
+- `timestamp` (cache timestamp)
+
+### When it is loaded
+
+`AuthService.hydrateFromStorage()` hydrates the cache at startup by calling
+`loadProfileCacheFromStorage()` in `js/state/cache.js`.
+
+### When it is written
+
+The cache is updated through `AuthService.setProfileCacheEntry()`, which:
+
+1. Writes the normalized entry to the cache.
+2. Persists it to `localStorage` (unless disabled).
+3. Syncs cached `name`/`picture` to the saved profiles list (if present).
+4. Emits `profile:updated` events for UI consumers.
+
+Common write sources:
+
+- **Own profile load:** fast relays, background refresh, or fallback in
+  `AuthService.loadOwnProfile`.
+- **General profile fetch:** `AuthService.fetchProfile` when a relay response
+  is received.
+- **Batch profile hydration:** utilities that call `setProfileCacheEntry`
+  (e.g., profile batch fetchers in list or thread hydration).
+
+### Cache reads
+
+`AuthService.fetchProfile` short-circuits to the cache if a non-expired entry
+is available and emits a `profile:updated` event with reason `cache-hit`.
+
+## Saved profiles list (`bitvid:savedProfiles:v1`)
+
+The saved profiles list is the persistent “recently authenticated accounts”
+store and is **not** TTL-based. It contains auth metadata plus cached
+display data to keep the profile switcher responsive.
+
+### Stored data (per entry)
+
+See `docs/nostr-auth.md` for the full JSON schema. At a high level, each entry
+stores:
+
+- `pubkey` (hex)
+- `npub` (optional)
+- `name`
+- `picture`
+- `authType`
+
+The payload also stores `activePubkey`.
+
+### Sync behavior from the profile cache
+
+Whenever `AuthService.setProfileCacheEntry` writes a new profile cache entry,
+it calls `syncSavedProfileFromCache` to update the corresponding saved profile
+entry’s `name` and `picture` if the profile exists in the saved list.
+
+## Comment thread profile cache (in-memory)
+
+`CommentThreadService` maintains an in-memory `profileCache` map scoped to the
+active comment thread. It is reset on thread changes and is **not persisted**
+to storage.
+
+### How it hydrates
+
+1. When a comment author pubkey is encountered, the thread checks the app-wide
+   profile cache via `getProfileCacheEntry`.
+2. If found, it is copied into the thread cache.
+3. If not found, the pubkey is queued for batch hydration and later written
+   back through the app-wide `setProfileCacheEntry` callback.
+
+This keeps comment rendering responsive without repeatedly re-fetching metadata
+within a single thread session.