Add Trading212 ticker normalisation

Context
-------
Phase 1 kickoff: Trading212 tags every ticker with `_EQ`, sometimes
preceded by a lowercase exchange letter ("l" = LSE) or `_US`. Raw
symbols like `VUAGl_EQ` are an implementation detail that would leak
into Wealthfolio and diverge from other providers (InvestEngine and
Schwab emit `VUAG` / `META`). The canonical form has to match across
providers so portfolio aggregation lines up.

Unlike the finance/ reference code, we do NOT restrict to a
SUPPORTED_TICKERS allowlist here — Wealthfolio is the source of truth,
everything gets imported, and the user decides what to track.

This change
-----------
- broker_sync/providers/trading212.py: pure `_normalise_ticker`
  helper backed by a single regex that peels `(_US)?[a-z]?_EQ`. No
  lookup tables — the rule covers all observed shapes.
- tests/providers/test_trading212_ticker.py: parametrised cases for
  every mapping called out in the Phase 1 plan plus pass-through of
  already-canonical symbols.

Test plan
---------
## Automated
- poetry run pytest -q  →  41 passed in 0.46s
- poetry run mypy broker_sync tests  →  Success: no issues found in 22 source files
- poetry run ruff check .  →  All checks passed!

## Manual Verification
Not applicable — pure function, no external side effects.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

broker_sync/providers/trading212.py

@@ -0,0 +1,15 @@
+from __future__ import annotations
+
+import re
+
+_SUFFIX_RE = re.compile(r"(?:_US)?(?:[a-z])?_EQ$")
+
+
+def _normalise_ticker(raw: str) -> str:
+    """Strip T212's exchange-suffix decoration from a ticker.
+
+    T212 tags every ticker with `_EQ`, optionally preceded by a lowercase
+    letter ("l" for LSE) or `_US` for a US listing. Peel those off so the
+    symbol matches what Schwab / InvestEngine emit.
+    """
+    return _SUFFIX_RE.sub("", raw)