# Task 04: Phone Number Normalization

## Objective
Build `internal/phone/normalize.go` — bidirectional conversion between E.164 and Philippine local format.

## Functions

### ToLocal — E.164 → Philippine local
```go
func ToLocal(e164 string) (string, error)
```

| Input | Output | Notes |
|-------|--------|-------|
| `+639171234567` | `09171234567` | Standard E.164 |
| `639171234567` | `09171234567` | Missing `+` prefix |
| `09171234567` | `09171234567` | Already local, pass through |
| `9171234567` | `09171234567` | Missing leading `0` |
| `+1234567890` | error | Non-PH country code |
| `` | error | Empty |
| `abc` | error | Non-numeric |

Rules:
1. Strip all non-digit characters (spaces, dashes, parens, `+`)
2. If starts with `63` and length is 12: replace `63` with `0`
3. If starts with `9` and length is 10: prepend `0`
4. If starts with `0` and length is 11: pass through
5. Otherwise: return error "invalid Philippine phone number"
6. Validate result is exactly 11 digits starting with `09`

### ToE164 — Philippine local → E.164
```go
func ToE164(local string) (string, error)
```

| Input | Output | Notes |
|-------|--------|-------|
| `09171234567` | `+639171234567` | Standard local |
| `9171234567` | `+639171234567` | Missing leading `0` |
| `+639171234567` | `+639171234567` | Already E.164, pass through |
| `` | error | Empty |

Rules:
1. Strip all non-digit characters except leading `+`
2. If starts with `+63`: pass through
3. If starts with `63` and length is 12: prepend `+`
4. If starts with `0` and length is 11: replace `0` with `+63`
5. If starts with `9` and length is 10: prepend `+63`
6. Otherwise: return error "invalid Philippine phone number"
7. Validate result matches `+63` + 10 digits

## Tests (`internal/phone/normalize_test.go`)

Test both functions with the table-driven test pattern:

```go
func TestToLocal(t *testing.T) {
    tests := []struct {
        name    string
        input   string
        want    string
        wantErr bool
    }{
        {"e164 with plus", "+639171234567", "09171234567", false},
        {"e164 without plus", "639171234567", "09171234567", false},
        {"already local", "09171234567", "09171234567", false},
        {"missing leading zero", "9171234567", "09171234567", false},
        {"non-PH number", "+1234567890", "", true},
        {"empty", "", "", true},
        {"with spaces", "+63 917 123 4567", "09171234567", false},
        {"with dashes", "0917-123-4567", "09171234567", false},
        {"too short", "0917", "", true},
        {"too long", "091712345678", "", true},
    }
    // ... run tests
}
```

Same pattern for `TestToE164`.

## Acceptance Criteria
- [ ] `go build ./cmd/server/` succeeds
- [ ] `ToLocal` handles all input formats in the table
- [ ] `ToE164` handles all input formats in the table
- [ ] Non-Philippine numbers return error
- [ ] Empty strings return error
- [ ] Non-numeric input returns error
- [ ] Spaces, dashes, parens are stripped
- [ ] Results are validated (length + prefix check)
- [ ] All tests pass: `go test ./internal/phone/`