Makes the event identifier more specific to Mostro to avoid
potential conflicts with other exchange rate providers on Nostr.

Updated:
- nip33.rs: new_exchange_rates_event() now uses 'mostro-rates'
- docs/NOSTR_EXCHANGE_RATES.md: all examples updated

Added exchange_rates_update_interval_seconds config option:

**Config (types.rs):**
- Added exchange_rates_update_interval_seconds field (default: 300s = 5 min)
- Added default_exchange_rates_update_interval() helper

**Scheduler (scheduler.rs):**
- job_update_bitcoin_prices() now reads interval from config
- Logs interval on startup: 'Starting Bitcoin price update job (interval: Xs)'

**Bug fixes (bitcoin_price.rs):**
- Fixed RwLock lifetime issue: drop lock before await
- Fixed Tags::new() → Tags::from_list()
- Fixed send_event() signature (pass &event, not event.clone())
- Use debug formatting for send_event output

**Documentation (docs/NOSTR_EXCHANGE_RATES.md):**
- Added exchange_rates_update_interval_seconds config example
- Documented recommended values (production: 300s, dev: 60s, low-volume: 600s)
- Added interval config to deployment checklist
- Warning about API rate limits for short intervals

**Example settings.toml:**
```toml
[mostro]
publish_exchange_rates_to_nostr = true
exchange_rates_update_interval_seconds = 300  # 5 minutes (default)
```

**Testing:**
✅ cargo check passes
✅ Configurable interval reduces API calls for low-traffic instances
✅ Default value maintains backward compatibility

Added expiration tag to prevent stale rates living forever on relays:

**Implementation (bitcoin_price.rs):**
- Calculate expiration timestamp: current_time + 3600 seconds (1 hour)
- Add Tag::expiration() to event tags
- Relays supporting NIP-40 will delete events after expiration

**Why 1 hour?**
- Exchange rates update every 5 minutes (default)
- 1 hour provides ~12x redundancy for client sync delays
- Prevents old rates from polluting relay storage
- Clients should always fetch fresh rates on app launch

**Documentation updates:**
- Added expiration tag to event structure examples
- Documented NIP-40 behavior in Relay Security section

**Code quality:**
✅ cargo fmt (formatted)
✅ cargo clippy --all-targets --all-features -D warnings (passes)
- Fixed unused variable warning in map_err closure

**Testing:**
✅ Compiles without warnings
✅ Expiration tag correctly formatted as unix timestamp

Implemented all inline review suggestions:

**1. Docs: Scheduler cadence configurable**
- Updated NOSTR_EXCHANGE_RATES.md to reference configurable interval
- Changed "runs every 300 seconds" to "runs at intervals configured by exchange_rates_update_interval_seconds"

**2. Docs: Add language identifiers to code fences**
- Added ```text to log snippet fences for markdownlint compliance

**3. bitcoin_price.rs: Dynamic expiration based on config**
- Expiration now calculated as min(update_interval * 2, 3600)
- Ensures events don't expire before next publish
- Caps at 1 hour to prevent stale data

**4. bitcoin_price.rs: Timeout for send_event**
- Wrapped client.send_event in tokio::time::timeout (30s)
- Prevents blocking the scheduler if relay is unresponsive
- Logs timeout errors separately

**5. scheduler.rs: Validate minimum interval**
- Added MIN_INTERVAL constant (60s) to prevent API rate limit abuse
- Logs error if configured value too low and uses minimum
- Prevents tight-loop spinning on misconfiguration

**6. nip33.rs: Updated example docstring**
- Fixed example to show HashMap<String, f64> (flat structure)
- Added comment explaining Yadio format
- Included expiration tag in example

**Code quality:**
✅ cargo fmt (formatted)
✅ cargo clippy --all-targets --all-features -D warnings (passes)
✅ cargo check (compiles)

**Note on rate format:**
Keeping current format (flat HashMap) as it correctly represents
"price of 1 BTC in fiat". CodeRabbit's suggestion to invert
(1.0 / btc_rate) would break the semantics — we want USD=50000
(1 BTC costs 50k USD), not USD=0.00002 (1 USD buys 0.00002 BTC).

BLOCKER 1: Unify content format across code and docs
- Fixed last occurrence in docs (jq example)
- Fixed docstring comment in nip33.rs
- All docs now show flat format: {"USD": 50000.0, "EUR": 45000.0}
- Consistent with code implementation

BLOCKER 2: Rename updated_at → published_at
- Tag name now accurately reflects semantics
- published_at = daemon publish time (not source timestamp)
- Updated everywhere: code, docs, examples, client spec
- Clarified in docs: "Unix timestamp when daemon published the event"

MINOR 3: Make publish_rates_to_nostr truly best-effort
- All error paths now return Ok(()) after logging
- Timeout errors no longer propagate up
- Comment added: "Best-effort: log errors but don't fail update job"
- Matches the "non-blocking" behavior described in docs

Changes:
- src/bitcoin_price.rs: published_at tag, best-effort error handling
- src/nip33.rs: published_at in docstring example
- docs/NOSTR_EXCHANGE_RATES.md: published_at tag, flat format examples
- .specify/NOSTR_EXCHANGE_RATES.md: published_at tag

Code quality:
✅ cargo fmt
✅ cargo clippy -D warnings (passes)

Ready for merge.

Changed content structure to match Yadio API response exactly:

**Before (flat):**
{"USD": 50000.0, "EUR": 45000.0, ...}

**After (Yadio format):**
{"BTC": {"USD": 50000.0, "EUR": 45000.0, ...}}

**Why:**
- Clients can parse it identically to Yadio HTTP API responses
- No transformation needed on client side
- Includes all currencies Yadio publishes (not filtered)
- Format is self-documenting (BTC wrapper makes it clear)

**Changes:**
- src/bitcoin_price.rs: Wrap rates in {"BTC": ...} structure
- src/bitcoin_price.rs: Updated tests to verify wrapper
- src/nip33.rs: Updated docstring example
- docs/NOSTR_EXCHANGE_RATES.md: All examples show Yadio format
- .specify/NOSTR_EXCHANGE_RATES.md: Client spec updated

**Testing:**
✅ cargo test bitcoin_price::tests (all pass)
✅ cargo fmt
✅ cargo clippy -D warnings (passes)

**Example event content:**
{
  "BTC": {
    "BTC": 1,
    "USD": 50000.0,
    "EUR": 45000.0,
    "ARS": 105000000.0,
    "VES": 850000000.0,
    ...all Yadio currencies...
  }
}

**BLOCKER 1: BTC key collision in test**
- Removed "BTC": 1.0 from input_rates in test_rates_json_serialization
- Now uses only fiat currencies (USD, EUR) like real Yadio response
- Added assertion to ensure no nested "BTC":1 in output
- Prevents test from normalizing invalid format

Why: Yadio includes "BTC" as the wrapper key, not inside the rates map.
The old test was creating {"BTC": {"BTC": 1.0, "USD": 50000.0}}, which
would never occur in production and could hide bugs.

**MINOR 2: Clarify Settings::get_mostro() double read**
- Added comment explaining why we read settings in publish_rates_to_nostr()
- Rationale: Ensures expiration stays aligned with interval if config reloaded
- Not a performance concern (settings read is cheap, publish is best-effort)

**Testing:**
✅ cargo test bitcoin_price::tests::test_rates_json_serialization (passes)
✅ cargo fmt
✅ cargo clippy -D warnings (passes)

Ready for merge.